IMHO, SONAR should follow a marketing strategy similiar to the one used for FL Studio:
1) Artist, Professional and Platinum should be sold exclusively with lifetime updates. Do you already own a version and want the better one? Simply pay the difference in price. No more problems/misunderstandings with the payment method.
2) In the lifetime updates should be included only the bug-fixes and the core enhancements while the future plugins (like the new L-Phase Series) should be sold separately: in this way Cakewalk could offset losses caused by the lifetime memberships.
3) Speaking about plugins already included: they're too many, expecially in Platinum. TTS-1, Cyclone, Dimension LE, Dimension Pro, DropZone, PSYN II, Rapture, Rapture LE, Rapture session, rgcAudio Square, Roland GrooveSynth, RXP, Lounge Lizard Session 4 and many more. Some of them I've never opened. IMHO, the only one that should be included is Rapture Session and the others sold separately, while the only third-party VSTs included could be Addictive Drums 2, Melodyne and TH3 (or Ozone 7 elements in partnership with Izotope): in this way Cakewalk could even reduce the price of SONAR.
4) The world is full of casual customers who only complain about the price or don't understand the membership method used by Cakewalk, just give a look at the SONAR discussions page on STEAM: with a slogan like "SONAR Platinum, only for 300$. Pay one time, keep it upgraded forever." sales would be guaranteed.