The way I see it, continuing to use SONAR is definitely a valid choice. It depends on what type of user you are:
Group 1If you're happy with your system's performance now, and don't plan any significant upgrades, hardware, plug-in or otherwise, you should be fine. Image your system drive, back up everything, keep your machine off the internets and you can be happy for a long time.
Group 2If you like occasional/frequent upgrades to your hardware (computing or audio interface, etc.) and trying new plugs or new features on existing plugs (Melodyne, Vocalign, etc.) you might be in trouble.
The problem for the second group of users is you don't know when SONAR will break, and you may have no warning at all. You can always go back to a previous drive image, but then that effectively puts you back in group 1.
Most of the Cakewalk developers are gone, the ones who agreed to stay through the shutdown are looking for work, and Gibson may go under at any time. So there may be no one to give you an activation code when/if the servers go down. There will certainly be no one to fix SONAR related problems with drivers/operating system upgrades. Something could break at any time or never.
I plan to keep SONAR on my system for a long time, but I don't plan to do any new projects with it. If it weren't so onerous to port projects from one DAW to another I would port everything and consider freeing up disk space by uninstalling SONAR, but I don't see a way of doing that efficiently.
Maybe another option is a dedicated SONAR DAW. Keep one older machine off the net with SONAR installed and have another machine for whatever other DAW you want to work with. Then you can be in both groups at the same time :)