ampfixer
THe only thing I can tell for sure is that it will cost me more to own Sonar than in the past, and that there will be no more free bug fixes or enhancements.
Realistically speaking, you ALWAYS paid for bug fixes and enhancements. When you spent $149 for an upgrade, that was factored into the cost. A company knows there will always be problems that surface after a piece of software is released, so you build in the cost of making those fixes.
New users get a year of free bug fixes, enhancements, and content. By the end of that year, the assumption - as it is currently - is that show-stopper bugs will have been fixed. Most users realize that software will never be bug-free and that X3 is substantially more stable than previous versions. By continuing to tweak that core, I would expect it will continue to become more stable in the future.
The way things work now, if there are remaining bugs, they may or may not be fixed in the next update. The way things will work in the future, if there are remaining bugs, they may or may not be fixed in the next update. However, the big difference is that QAing will be easier, and the modular installer makes it easier to do what used to be called "quick fixes."
It's costing me more and you have a cop out option for buggy updates. You will get the $149 from me so that I can experience the new format for a year and then I'll evaluate the experience. It doesn't feel good, but you're the dog and I'm the tail.
But again...that's no different from now. Suppose you had X2. You paid $149 to update to X3, even though maybe you were nervous about new ownership and thought the future might be uncertain...but you took a leap of faith. Then you received five updates over the course of six months, enjoyed more stability, found out all the cool things you could do with X3, and then you were in a position to decide whether you thought the $149 was well-spent or not, and whether you'd update to X4 when it appeared. That's what's happening, EXCEPT you're getting what would have been X4, while assembling "X5" a block at a time over the next year
and using "X5" features as soon as they're ready.
Personally speaking, I think the improvements in "X4" are well worth $150. Mix Recall is huge, so is VocalSync and the MIDI enhancements. The Control Bar is now fabulous...and of course, the new amp sims

That's not all but for the way I work, those would justify $150.
It has always been your option to say "Nope, not gonna upgrade this time around" and if you didn't, then you weren't going to get bug fixes for the version that was no longer being sold. I really can't think of any major music software companies that continue to offer new features and bug fixes for versions that are no longer being sold, although there probably are.