"Mozilla did kill XPCOM" isn't a deviation from their stated intent. XPCOM was a magnet for Hyrum's Law problems, because obviously XPCOM plugins are going to depend on the inner workings of the browser, that's just how XPCOM is designed - so now if you touch these internals it breaks third party stuff. There were operating systems which took the approach XPCOM has to extensibility, they're not doing so great: Classic Mac OS, the Amiga Workbench, MS DOS... That's just not a sustainable situation, Mozilla had to kill XPCOM.

So to the extent Mozilla failed to deliver here it's on the replacement APIs. But how much is enough?

I would like lots of things to have APIs that don't. For example I'd like a way to do some basic queries on the built-in Public Suffix List for Firefox instead of needing to either bake the PSL into each plugin (and keep it up to date) or call out to a web API (ugh) or just guess that TLDs are "enough" and make everybody who needs other suffixes mad.

But in that particular case there are two reasons we don't have such APIs. #1 Nobody did the work. I didn't do the work, you didn't do the work, the work didn't get done. #2 In many cases (I think not mine but it's always arguable) the PSL is the Wrong Thing™ and so encouraging more use of the PSL makes things worse.