It's worth noting that those numbers are partially because of licensing, not just technical constraints.

All of the original publishing contracts allowed Microsoft to sell Xbox/Xbox 360 games on those systems specifically, so Microsoft had to seek permission from each and every relevant third party publisher before making each title backwards compatible.

From what I understand original Xbox compatibility on the 360 was similar, with 462/996 games ultimately being compatible "officially" (https://en.wikipedia.org/wiki/List_of_Xbox_games_compatible_...). But with a jailbroken system compatibility is actually a fair bit better (https://consolemods.org/wiki/Xbox_360:Original_Xbox_Games_Co...).

Afaik there is no similar jailbreak for Xbox one/series, but if anyone ever does manage to get their internal emulator running with arbitrary titles I wouldn't be surprised if a lot more games work than their official numbers suggest.

I don't understand why contracts don't include provisions that allow Microsoft or other console manufacturers to emulate or have the game run on future consoles. Surely it'd be in the publishers interest to keep the game alive?

publishers want to resell the games at full price. your copy from 2002 still working conflicts with that motive. hell, they don’t even want to sell it to you these days. they’d much prefer you rent.

I work at Microsoft but have no inside knowledge. Presumably that wasn’t a consideration at the time? I suspect it is today, since games released for Xbox One can also be purchased on Series S/X…

What happens when you weight it for sales numbers? Personally I've only ever had one 360 game not be compatible with my xbone.