8-10 years is what the Consumer Rights watchdog expects for devices in the price range of a MacBook in my nation to continue to provide hardware support, so I would say that seems like a reasonable amount of time for there to continue to be software support.

MacBooks aren't cheap. For the average person they are a significant expense. So the lifetime of support should reflect that.

The thing is, this is in areas that worked fine for over a decade. Somebody must have decided that Xcode's text input code (that's one of the things that got slower) needs to be rewritten for no obvious reasons, but then didn't optimize it as much as the old code.

What's the point of new hardware if the new software that's written for it makes it just as slow (or in this case: much slower) than the old hardware?

not OP, but for the lifetime of the hardware ? or, at least, should be specified in T&C when buying the hardware

I am still pissed off that my 2012 Macbook Air is no longer receiving OS updates - compare that to Microsoft that allows me to run windows on basically any PC

How many years do you expect?