The standard is not whether something worked a year or two ago, but whether it followed the recommended best practices in effect at that time.
The important thing is always whether something works properly. Everything else -- formal standards, compatibility work, portability work -- is just a means to that end.
If you write a site using standards on the verge of being declared obsolete, you have no one to blame but yourself.
Which is an easy argument to make until someone points out that in these cases the people declaring something "obsolete" are frequently biased and, in particular, advocating a new and inferior replacement.
Dependence on NPAPI plugins hasn't been best practice for a long time now, much longer than one year
And yet viable alternatives to the things we've been doing successfully with various plug-ins for literally a decade or more have barely been around that long, and in many cases are still obviously and objectively worse in significant respects today.
Flash is the only plugin with any widespread support left, and it's been on its way out for a while.
Not in corporate use. Not even close.
Sites which depend on such plugins already fail on mobile browsers, which are becoming more and more popular and haven't even supported Flash for several years, much less other plugins.
And the corporates mostly don't care, because they have real work to do and provide their staff with real computers to do it. No-one is preparing their quarterly accounts presentation on an iPhone.
Plugins, on the other hand, have always been a compatibility nightmare—non-standardized, proprietary, and non-portable.
And yet Java applets were recognised as early at the <applet> tag somewhere back in the 90s, while Flash has been one of the most successfully standardised parts of web history in terms of both portability and longevity. I suspect only HTTP, HTML 4 and CSS 2.1 have been more successful in those respects.
If you like standards and cross-browser compatibility, you should be backing this change.
I like things that work. To be fair, I also like the new "standard" and "cross-browser compatible" features, but for a very different reason: they are still so badly implemented so often, and broken so often by browser updates, that I make an awful lot of money fixing things that rely on them.
fewer one-off, closed-source, browser- and OS-specific binary plugins
Because ECE for multimedia playback and graphics drivers to accelerate WebGL are so much better?
It really isn't, in any practical sense. Realistically, Microsoft are going to continue supporting it until at least 2020 because of the Win7 support, and because dropped it would cost them the support of the business community that makes up the lion's share of revenues.
For perspective, that is more than 30 six-weekly update cycles of various other major browsers where businesses don't have to worry unduly about something they rely on being arbitrarily broken.