I'm not saying that there's a limiting factor now, but Apple can't promise that they will never make a swift to a new connection system, because that day may happen in 10 or 15 years. I don't see Apple limiting new features (like the patented smart bands) in the future just to keep some people happy, there will be a day when Apple will change this band connection system, and they will need to be sure it's the good move to do precisely because people will not be happy.
The hypothetical connection system isn't the issue. You don't need backwards-compatibility for existing dumb bands - they're not going to get any smarter. The only question is whether, whatever that Smart Band system may deliver, can it be accommodated within the current physical constraints?
One thing you can be sure of is that 10 years hence, whatever physical or wireless communications-and-power method that will be necessary for a Smart Band will be possible
within the current physical constraints. There's a certain inevitability that the future will bring greater miniaturization, not less, and more/faster wireless communication, not less. Wireless, of course, need not be constrained by the band-fastening system at all, although to keep power requirements/emissions to a minimum it would make sense to embed the antenna within the band fastener, as close to the body of the Watch as possible.
A recessed diagnostics port with metal contact pads existed in Watch from Series 0 through Series 6. Although the number of physical contacts changed between the six-contact Series 0 and the five contacts of Series 1-6, the location and necessary functions (the Series 0 had a "dead"/unused pin) remained the same. For obvious reasons those contacts were covered by an access hatch. A future Watch model simply needs to substitute flush-mounted contacts (like iPad's Smart Connector) et voilà!
However, Apple seems to have gone wireless with Series 7
https://www.macrumors.com/2021/10/13/apple-watch-series-7-lacks-diagnostic-port/ , so the need for physical space in the band slots has vanished.
Now, a Smart Band would need to be powered (we still won't have perpetual motion machines in 2031). That future Smart Band (and Smart Band-supporting Watches) could have a simple, two-contact system for supplying DC power to the band - that would require
less space than the old diagnostics port. Or some sort of wireless charging method could be used, such as a charging cradle with a second set of charging coils aligned with charging coils in the Smart Band. And why suppose the band would be powered by the Watch battery, rather than the other way around? A Smart Battery Band would likely be more popular than any sensor-studded, data-gathering band.
One thing of which you can be certain... If I can brainstorm this in a matter of a few minutes, imagine what real engineers can accomplish over the next 10 years!
There's also the question of just how much data would need to be transmitted between band and Watch body, and at what speed. The Watch diagnostic port had a similar configuration to USB-A 2.0 (up to 480 MBPS) - a two-wire serial data channel, +/- DC power, and an ID/control pin. Bring it up to seven/eight contacts and you have Lightning or USB 3, which both have a pair of two-wire serial data channels. If you want to up the ante, 16-pin Thunderbolt has a pair of 4-wire data channels (20 GBPS per channel). Two such channels are provided in a Thunderbolt port, four in 24-pin USB-C, but in a proprietary system just one channel may be necessary. So just how much data has to be collected/transmitted via that Smart Band??? Assuming the body of the Watch itself will be able to pack in more sensors and an even-more-micro-miniaturized SoC by then, what capabilities would be so bulky as to need to be outboarded to the band?
One application is medical monitoring, such as a blood pressure cuff. However, while band-embedded sensors may be necessary for that application, the data requirements are not very heavy.
Thunderbolt speeds are primarily required for high-speed mass-data storage/retrieval, or high resolution graphics. What are you expecting in 10 years, the equivalent of a wrist-worn MRI/CT/PET scanner? You look at the large, bedside medical monitoring equipment of today - that's likely to be the maximum of what will be wrist-worn in 10 years time. How many sensors/electrodes are required by an EKG? EEG? BP? Temperature? Blood chemistry? How much of that requires high-speed data? Very little of it.
More interesting (and much more data-intensive) is the field of prosthetics - sight, sound, touch, scent/taste... all require far more data/processing than the typical bedside biophysical monitor. However, it seems likely those would continue to use dedicated, embedded systems rather than be outboarded to a general-purpose computing device like a mass-market Watch - especially a
removable processor/communicator. And it's also likely that Watch would not be the vehicle - this seems more like a job for eye/ear/nose/brain-adjacent Apple Glass.
I have a family member with a fairly simple implanted medical device. Every few months its parameters need to be tweaked/reprogrammed, which requires a wireless "communicator" held over the skin near the implant. That in turn is connected via Bluetooth to a smartphone/smartphone app. A bit kludgy, but it works. Apparently, Bluetooth isn't sufficiently powerful to punch through an inch or two of skin, fat, and muscle, so that extra communications device completes the "last mile." Or maybe it's just a matter of upgrading the implants from proprietary to Bluetooth?
That family member also has MFI (made for iPhone) hearing aids - no intermediary device necessary, just direct Bluetooth connection between hearing aids and iPhone for parameter control, audio (direct audio for phone calls, music, etc., plus access to the iPhone's microphones to extend the "reach" of the hearing aids' built-in mics), and not-quite Find My (Bluetooth homing). So who knows what the future will bring (although again, Glass seems a better vehicle than Watch for hearing or vision-related prosthetics)?
Yeah, we can never know the future. You can be 100% correct - Apple could ditch the current bands for some really whiz-bang capabilities in future smart bands. From my perspective... so what? If the capability is sufficiently stupendous and compelling (or medically necessary), people will say goodbye to their collection of dumb bands. Or they may have a choice of sticking with the existing standard on a bare-bones Watch. Apple Watch SE vs. Apple Watch Pro. You think Apple is incapable of such things? ;-)
Well, back to the Star Trek Universe (aka Days of Futures Past/Past Futures... whatever)... Just how long ago were TV audiences introduced to the Medical Tricorder? Hand-held communicator (1964, long before the flip phone)? Tap-to-call, compact and stylish Com Badges (TNG - 1987-1994). Geordie LaForge's prosthetic goggles (again, TNG)? And what about Dick Tracy's Wrist Radio (1946) and Wrist TV (1964)? Damn, we were still in the vacuum tube era for the Wrist Radio, and the discrete transistor era for Wrist TV (although still CRT for displays) - neither were practical until today's SoCs. AT&T Picturephone (not PicturePhone, as the intercap had not yet been popularized) debuted at the 1964-1965 New York Worlds Fair. All this is to say that the future doesn't arrive quite as quickly as we like to think. The currently-inconceivable tends to be many decades into the future, not just one.
Overall, it's reasonable to expect that our current bands will either wear out or become unstylish long before Apple has a tech-driven need to change fastening systems. And, of course, there's Creative Destruction. If Apple Glass is MacRumored to replace iPhone in 10 years, why should we expect today's iPhone-tethered Watch to persist?