java on the web is super niche these days, so as usual you have no idea what you're talking about
As usual you're talking out of your backside (I'll leave your equally usual problem of missing the whole point to latch onto a relatively minor slip-up aside).
It might be niche, but a very important niche (IRS, government apps, program licence updates, etc., etc.).
Browsers run JavaScript (I sometimes mix up the two), but that doesn't mean Java is "super niche". It is niche, but you'll still run into it pretty often due to that niche being rather important. Breaking it would still mess up a some important sites, even if it wouldn't crash the browser itself. Besides, Java libraries are only one place you can find this kind of shoddy programming. I bet it's not the only place if MS decided to take it into account.
I wonder if patching those programs is so hard to do, but still, wouldn't a better option be to have the build be something like 8.7, the name be something like "Windows Nine", and only in bitmaps and so on would it appear like "Windows 9"?
We're talking government apps here. Getting government to fix
anything is hard, at least where I live. "Windows Nine" could be a good idea, though. It's odd that MS didn't think of it (or maybe it did, and rejected it. Remember, they are a corporation afterall...). It'd avoid this problem while being less weird than skipping a number.
As for the build number, it's not really tied to "Windows number". IIRC, for Win 8.1, it was actually 6.3 (with Vista being 6, 7 being 6.1 and 8 being 6.2). Build number is what should have been used in the first place - it's the only proper way to identify which OS you're actually dealing with. And yes, it'd be really, really funny if the "next generation" Windows 10 gets a version number of 6.4...