Good riddance, I guess? I believe there isn't any shortage of good PDF viewers for GNU/Linux.
PDF is only open in the sense that .docx and other MS Office formats are open - there is a spec, and you are allowed to implement it, but it is almost impossible to implement 100% of the features. Programs like Evince (Poppler) and Sumatra have come a long way. They are really excellent and read 95% of PDFs, many even faster and with less RAM usage than Adobe, and almost 100% of static PDFs ("print to PDF").
But for the occasional PDF form, if I don't want to print it and fill it out by hand, I have to use Adobe (Poppler sometimes gets the font size wrong so you don't see when you are overflowing a field, code that checks whether you filled it out correctly doesn't work, and highlighting that tells you which fields are for what kind of user to fill out is also often messed up).
Actually what I just do is to run Adobe Acrobat Pro in wine, which allows me to actually save arbitrary filled out forms.
Finally, one ironic benefit of the Adobe Reader is that it is distributed in binary form. If you are stuck on a Linux distribution a few versions back, you can just download and extract it to your home directory and run it from there. I realize that from-source or package manager purists might cringe at that thought, but it actually saved me a couple of times when I just had to print out a stubborn PDF (and couldn't ask the admin to kindly update my distribution at work...).
this is actually accurate (for ex how many know that the reader has a Javascript execution engine integrated into it?) - when people say that there are other free 'alternatives' anyway, most miss out on this fact. Like you mentioned, good alternatives cover 95% of the use-cases, so almost no one discovers this by hitting a wall.
I always treated that as a bug, not a feature, and prefer non-adobe readers even on Windows.
For example UK HMRC tax returns must be done using Adobe Reader.
Just because Linux is open source, doesn't mean every app on it has to be. It needs popular apps to ever be a real desktop OS. No different than why Windows phone OS struggles today.
No, but every app being open source (or rather Free Software, which is usually the case) can be supported easily by communities of users who can then bring it to tons of different architectures. That's why you get Evince on ARM, for example, and not the official Adobe Acrobat Reader.
The problem is not unique to open-source software.
I've been bitten this way too. It was a hardware/software product, small company made a neat little USB DAQ that became very popular. Bigger entrenched player bought the company and discontinued the line. Shortly after that they released their own product based on the ones they discontinued, but now it requires a much more expensive software package. Just because a commercial product is profitable does not guarantee its longevity.
Virtually every open source application is also Free Software, and vice-versa. The most common licenses (GPL, BSD, MIT, Apache, etc) fit both definitions.
GNU's not Windows, nor strives to be one. It's about bringing Free Software to whoever wants/needs it, not winning in popularity contest.
There's a lot of linux-compatible closed source apps I enjoy. Teamspeak, games, etc. Adobe Acrobat is one of the ones we absolutely don't need - there are many better alternative.
Vendor lock-in?
I prefer Okular or Evince for Desktop viewing and pdf.js for in browser viewing. pdf.js is a bit slow when loading the pdf. But the user interface is quite nice in my opinion.