I already have a Studio Display plus Mac Studio, so this news doesn’t affect me at all. I still have numerous old iMacs taking up space in my garage. I would be more likely to buy an iMac again only if they offer target display mode again. I understand why they didn’t before because they had to come up with a customized video controller since Thunderbolt 3 didn’t exist at the time. They should be able to do so now.
Before Thunderbolt the "Target Display Mode" primarily worked on a DisplayPort (DP) port. They didn't need a 'customized controller' as much as a DP switch(es) which Apple could toggle to reroute the port from being a 'display output' port to some path inside the system back to the embedded display.
GPU out 1 --> | DP | ---> DP to embedded screen |
GPU out 2 ---> | routing | --> DP port out mode |
| switching | <-- DP port in mode |
( center column above represents the 1 or 2 switches )
So either GPU out 1 or DP port in drives the screen depending upon how the switch to route the signal. Similarly GPU out 2 goes out the DP port out is blocked by some signal coming in.
The 5K display presented a problem because there wasn't evenough single DisplayPort v1.x bandwidth to drive the display. So needed two signals to drive the display and a customer display controller (that takes two inputs ).
Thunderbolt 3 only got to point were could carry two signals over a single wire. However, it doesn't inherently solve the issue though. These are are some TBv1 diagrams but present the core problem in the modern context.
A year after its debut on Apple platforms, Thunderbolt is finally available for PCs. Both fast and scalable, the technology’s 10 Gb/s connectivity and potential for external graphics promises to inspire innovation. But is it ready for prime time?
www.tomshardware.com
Later TB controllers took two DP ports in and had ability for two DP ports out. Historically, this was done with discrete controllers. Some variants of the Intel controller could be used in both "Host Mode" (inside a PC) and "Endpoint Mode".
Two layers of problems. One is that the 'target mode' for display needs to switch the controller from 'host' to 'endpoint' mode. The other major modern problem though is that "host mode" is primarily not implemented using a discrete TB controller. There are no "DP out" ports on an embedded TB controller. Intel mobile and all of Apple's TB controllers are now embedded onto the main SoC die (or at least onto the main SoC chip package). It is a big Perf/Watt boost and economical at scale.
This is all very similar to the USB 2.0-3.0 world where most mainstream host USB controllers are integrated into the rest of the platform provisioning die; not a discrete chip. And peripherals are made in such large quantities that endpoint only USB controllers dominate. Thunderbolt merging with USB-IF means that TB controllers are extremely likely to diverge the same way USB controllers have.
Myopically focusing on TB's bandwidth to solely solve the problem is misguided. If there is no path to get the signal to an embedded screen ( no DP out port) then TB won't be a solution.
Could Apple go through gyrations to embed not only a TB controller but also a relatively very rarely used DP switch on the die ? Technically possible. But relatively very expensive use of die area when there are lots of other die subsystems that can deliver greater bang-for-the-buck.
If they don’t, I’ll just stick with my current setup and replace the computer or monitor whenever one of them gets too old or breaks. I like that my computer and monitor are separate since I don’t have to throw away a perfectly good monitor whenever I want to upgrade the computer portion.
More than decent chance Apple's 'Green Team' is pushing for that approach also. Laptops get a 'pass' because integration is an essential element of being able to take the 'whole system' with you. iMac 24" probably gets a pass because now a much narrower part of a display modular desktop line up. There are some device security contexts where one whole system works better, but not looking to make that Apple's 'king of desktop volume' model.
If a larger screen iMac ever comes back it would likely be relatively low volume (and much higher margin) and targeted at a narrow niche. If Apple is trying to push more of their products to 'zero carbon footprint' lifecycle then one easy factor is to greatly end the life. For monitors that is much more easy for them to do. Apple doesn't refresh monitor quickly before. ( e.g., sold the 30" display for 6-7 years. sold the TB display for 6-7 years , etc. )