Having built some SPA's, I'd argue that at least those two things that you mentioned are the fault of whoever built bad SPA's, rather than issues with SPA's as a whole. It's possible to build an SPA that works well with the browser controls because modern browsers expose a lot of their functionality to JavaScript (like it or not). You can use <a> tags for buttons and links in an SPA (and keep the link functionality) -- you can just intercept the clicks when the link is being opened in the current tab, and you can always hook onto pushState/popState to make the back/forward button work.

It is an issue with SPAs as a concept because part of that concept is that you’re taking on responsibility for things the browser traditionally did for you.

That’s not a bad thing but it’s extra work which someone has to be responsible for, similar to how e.g. using a complex custom widget means you’re on the hook for accessibility rather than the browser vendors.

Surely most creators of SPAs are using some routing library, and I’m not aware of any routing libraries that don’t handle this anchor tag click behavior by default. It’s really a trivial amount of “responsibility” to take on.

Perhaps, but it’s something I encounter fairly regularly, especially when an unreliable network connection or other JavaScript failure breaks something with poor error handling. Again, not a showstopper but it’s a decision point: if you aren’t committed to testing this kind of thing on every release, you don’t have the resources to build an SPA.

It's no different than stuff like not trusting browser input or avoiding a massive session when you're working on a Web app: it's simply basic best practice stuff that you don't even have to think about if you have an idea what you're doing.

Would you say Youtube works better now as it is fully SPA?

There's an army working on YouTube, and it has the traffic levels to justify the investment. They can actually put in the required effort to ensure the application functions seamlessly and meets accessability standards.

It does not. The SPA is so sluggish I’ve learned to just force a page refresh after clicking on anything because a full page load is actually faster than their bullshit SPA.

That's exactly the reason to use a SPA framework where all these problems are solved for you.