I wouldn’t say “easily” avoided. You can’t ignore the billions of web pages that would have already existed in a format that was non-compliant with XML at the time. With so much “prior art”, there is simply no way that any browser will ever be able to throw out its fuzzy/imprecise parser, which means that support for well-formed XML requires them to maintain two readers: precise and imprecise.

As far as XML “tools”, I am shocked at how even now I encounter real XML parsers that don’t necessarily reject malformed data files but do atrocious things with them (like silently pretend that certain tags were not even in the file). Thus, I end up using extra steps like a linter as a front-end sanity check. And while this example is a pure-data application, a linter is also a sensible front-end sanity check for HTML. XML isn’t going to win over HTML if it requires the same steps to clean up imperfections in the process.