> Surely lack of hardware acceleration and poor implementation are incidental, not fundamental issues.
Of course not. These are just some of the many many needles of crap that broke Flash.
> I expect the same would have happened for Flash eventually.
Perhaps you underestimate just how complicated "Flash" is: It's 2023 and despite everything you can do in a modern Web browser with HTML5, SVG, JavaScript, Video and so on, we still don't have a second full reimplementation of Flash. No emulator or converter that preserves all of the authors intentions, not even with a server-side-helper to cover the differences in networking policies between the web and Flash. I still think it would take serious cash/time to do this.
And for a large company to do it would risk being sued by Adobe. They promised. I think if Adobe couldn't fix the crap-needles for whatever reason, nobody else could either. Adobe made sure of that. And nobody wants to pay the Adobe tax when Flash hurts users so bad.
PDF on the other hand, is easy enough to implement on screens in a month or less, spec-in-hand. Users who use Adobe's PDF reader deserve what they get.
> We have WebKit and Gecko, and the latter barely exists on mobile. I'm not convinced we're a lot better off in practice.
I do feel a little better off. I remember a time when every flash bug was an opportunity to airdrop malware, just buy an ad and get on every desktop PC in the world for chump change. I had to browse the Web in a VM, back when VM's on workstation PC's were still painfully slow, because I needed the ability to rewind state so often. I really don't miss that.
A small number of players working (largely) openly (i.e. we have webkit and gecko's source code!) allowed features to the Web be deployed relatively quickly, and problems fixed fast and usually with the smallest-possible harm. And I think people are sufficiently suspicious of closed-source infrastructure that people (mainly purchasing VPs at big enterprises) will be able to resist any attempts to change that.