I think it will be possible to keep a usable version of SONAR running for some time. Abandoning the software is unnecessary for the average hobbyist who can afford to keep a zombie system running. For a music professional, the argument is pretty much the same, if you are making and recording the music you want with the current version, you can keep doing that. You already had worked out ways to kludge your stuff into a clumsy compatibility with other DAW users or studios and those workarounds will probably still work.
For the recording professional, especially one operating a paying studio based on SONAR, the question is probably not whether SONAR is usable, but rather if it is any longer salable. It is very common when looking through ads and listing of recording studios to find the proud banner that this is a ProTools studio. That was never worth the price per word for SONAR users. I would hate to be the operator of a SONAR studio trying to convince his potential customers that SONAR is as good as or better than ProTools now, and trying to convince them that their projects can be loaded and tweaked a decade from now. I would expect there will even be some old customers who will want their projects ported to some new DAW once the word gets around.