In short,
1. You're paying for an upgrade, just as if Sonar X4 were released
2. You're getting a whole extra year not only of bug fixes (used to be roughly half a year) but also of extra features (we never had this before).
3. You're not getting cut off after a year, you get to keep your software as is. Again as it was, except now you'll have a newer version.
4. You don't HAVE to upgrade after a year, you can wait until new features become available that you're interested in (no different from before).
5. Cakewalk will have to provide some incentive for people to upgrade right after their year is up, so I'm personally guessing/hoping there will be some discounts for those who wish to continue with no interruptions.