We could do it if we had to, of course.
Military could probably pressure Microsoft to give them some way to turn an original xbox into a PC; and if you give people $50 for an original xbox most will be happy.
As an end user it would be manageable but dont forget that the whole world runs on servers and many Industries would collapse if computing power is gone
If we lost a few decades of processor improvements, it would be about as parent noted.
If we lost worldwide connectivity or all undersea backbones between some terrestrial networks... it would be civilization-altering.
On the server side, you'll be running C code rather than a nice high-level language, but it will get the job done. You might not have all the ML which lets me get the most relevant product recommended, but....
Emails definitely won't be HTML, but they'll come through.
The world would work.
On the whole, that 1000x increase in performance mostly bought the ability to have most of the code that runs be interpreted or JITed rather than statically compiled, to play videos, to do some heavy numerics and graphics (including things like family photos and videos), and so on.
It's hard to think of anything I really /need/ in a life-or-death sense or an economic-survival sense that couldn't be done on older hardware with appropriate software. There's a ton of things I want, that help, or where migrations would be massive projects.
If computers were to entirely disappear, and I couldn't automate things, communicate digitally (emails, messaging), word process, write code, I think that'd be a major systemic-collapse-level implosion of society. A lot of people would simply die.
On the other hand, if computers were to regress to 33MHz-level performance, Youtube isn't sticking around, but we'd likely adapt as a society with some structural change, but without such a collapse.