The sky isn't falling.
I am (and always was) one of the more bitter skeptics of many things Cakewalk does. With the benefit of some years behind me, it's safe to say, many of the things I was skeptical about are starting to pan out nicely.
* I was skeptical about the rolling update model, but it worked.
* I was furious about the unreliability and bugginess of the DAW, but much of that is in the past now.
* I was disappointed with the ergonomics of the DAW, but with some tweaking, it's actually well ahead of the competition. And oh yeah, the performance still kills it.
So, what do the lifetime updates mean from a cynic's perspective? The DAW is becoming more and more commoditized and more and more serves as a marketplace for other commercial products. LANDR, Soundcloud, Gobbler are already being sold through SONAR and other DAWs in addition to all the instruments and effects. I expect more will come.
Thus, a user who keeps using Sonar and never pays for another update offers far more revenue than a user who buys Sonar and never uses it again, even though they have paid the same number of dollars out of pocket.
This suggests a couple of probable outcomes: while lifetime upgrades are currently being offered only for Platinum, this is likely a push to sell upgrades rather than a true limitation. I suspect we'll see lifetime updates for other editions in a while. Also, while the lifetime updates are likely marketed as a limited-time offer, I think it's likely that they will either not disappear or eventually reappear. Because Cakewalk has an incentive to keep its customers up to date if only so that they can provide the next built-in integration with a commercial third party service.
(Edit: grammar)