Did a computer bought to run windows 95 run XP? Did a computer bought for 98 run Vista? That’s a more fair comparison, as mobile operating systems are very young. And mobile devices from 10 years ago have hardware that could not really be compared to computers.
Sure, processors at peak capacity where good. But forcing a 10 year old processor running todays software would drain the battery - that was also in no way comparable to today - to fast. And that is even if you could install the OS, as there is so little device space on many of them. Then you open one app and you’re out of ram potentially causing crashes all over the place, because mobile apps are rarely built for efficiency.