Yeah, historically that's been one of the issues, along with relative resources used (CPU/RAM/storage) and especially going back to the iPhone launch days with pretty much everything touch(virtual keyboard is still keyboard input, so not 100% but most), something like IOS made a lot of sense, with the iPad coming not much later.
At this point in time, however, enough things have been 'cross-ported' and at it's core, I suspect a lot of macOS internals were still used as the basis for IOS/iPadOS. Meanwhile we can run iPad and IOS apps on macOS now, so it's not 'all done' but it's significantly less work compared to initial iPhone/iPad/IOS launch.
In reality, there's no need from the forced 'run macOS if a keyboard is connected' bit. Touch just becomes another input layer, just like for example, I don't need to select 'make my trackpad OR my mouse active' or 'disable my trackpad whenever I connect my mouse, or when I connect my external keyboard, or display, etc.
Once touch input is completely solid on macOS they could do something like a simple control panel toggle to switch across UI and even 'behavioral' styles (iPadOS or MacOS) without too much effort, and if someone wanted to they could for example, script or have native options for docked, undocked, whatever, and there's no need for it to be 'forced.' There's additional code, but should not much added overall compute required, as switching UI/behaviors is effectively a 'run these code paths and not these' vs 'all the time running both.'
Yeah, there's no reason for forced modes if they do a bit of optimization (e.g. MBA can still thermally throttle and has more thermal mass to help in passive cooling) and perhaps downclock a bit on the iPad side of things.
There shouldn't be anything truly 'forcing' one or the other. It's entirely possible to see some models (e.g. mini, smaller iPads) continue running IOS only, and Pro or another tier, 'Ultra' to be consistent with Apple's chip naming anyways to be running macOS..or the option for 'macOS UI and behaviors' anyways.
It would open up a bunch of options for others without negatively impacting anyone that wants 'an iPad to be an iPad and a Mac a Mac' or whatever...if done reasonably.
An interesting thing may come out of such a change on the revenue side. The immediate thinking would say - would negatively impact MacBook sales. It might, but it may not affect overall revenue or even increase it.
I literally never use the trackpad on my MBPs except when mobile as it's on a stand with screen open next to my primary display, and I'm also not convinced it's better for RSI-type issues vs my trackman marble clone. However, if they enabled pencil on trackpad - for the first time ever, I'd be buying a Magic Trackpad and another pencil.
If I didn't need to include portability on my primary work system (MBP, long history of 15"s, then 2019 Intel 16", now MBP 14), I'd either shift back to the more expensive 16", or quite possibly move 'up' to a Studio Ultra, all of which is net additional $$ to Apple. I might well also consider, once the iPad 'Ultra' is more useful to me beyond consumption and basic email/docs, a more frequent upgrade path on my iPads, as I would see actual benefit vs my 2018 IPP 12.9" that gives zero reasons for upgrading due to iPadOS capability limitations for what I do.
