Please correct me if I have these dates wrong:
 
Sonar X3 was launched in September 2013.  Its last update (X3e) was March 2014.  Therefore, its "lifetime" support (in terms of updates) was effectively 6 months.
 
Sonar X2 was launched in September 2012, and last updated in March 2013 (again, 6 months).
 
Sonar X1 was launched in December 2010, and last updated February 2012 (14 months).
 
The entire "X" series lasted from December 2010 to March 2014, which was 3 years 3 months.
 
Sonar Platinum was released in January 2015.  After about 1 year 5 months Cakewalk [apparently] realized they weren't making enough money from the subscription model, so are going back to the "lifetime updates" model.  Apparently.
 
To calculate whether the $199 "lifetime deal" is worth it, we need to estimate how many more months or years Sonar Platinum will be actively developed/maintained.  It if only lasts as long as X did, then we have less than 2 years left, so $199 is not worth it (compared to $99 per year, especially for those of us who have a lot of time left on our current subscription).  I guess we just have to cross our fingers that Platinum lasts [significantly] longer than X did.
 
Of course, it would be very frustrating if we pay for Platinum's "lifetime" updates now, and then a year or two from now, Cakewalk retires Sonar Platinum and instead releases a new product (which doesn't count as an "update" and therefore requires us to pay all over again). 
 
Or. what if Cakewalk starts deliberately packaging updates with paid add-ons?  "Sorry, but to get that Piano Roll bug fixed, you have to buy the new version of Dimension Pro."  Or even "If you don't like the current piano roll, then buy our new Advanced Piano Roll plugin, for only $19.99".  In other words, micropayments.
 
What are Cakewalk's options?  Can they really afford to give us free updates every year, for 5, 10, 15, 20 years?  With no requirement for us to ever pay them again?  Hmm...
 
Until now, Cakewalk has gotten about $100 a year from me, just on Sonar upgrades.  Something tells me it's EXTREMELY unlikely that they will accept 2 years' advance payment and in return enslave themselves to us for the rest of their (and our) natural lives.
 
So how will they survive, as a company?  Will they resort to ads, embedded in parts of the Sonar UI?
 
What road will Cakewalk take?  And when?
 
What is Sonar Platinum's life expectancy?

Any guesses?

I would suspect Marketing will wait until they have OSX Sonar running solidly and is comparable with Win Sonar. To help achieve this process they may need an injection of cash (assumption) like from lifetime payment. The extra cash could be used to hire in OSX programmers, outsource OSX coders, buy in code from 3rd parties for aspects of OSX version.

Once they have two operating systems running they will release Sonar "Diamond" and by having both win and OSX they can get good marketing leverage in mags and shows.
 
Just one of many applicable theories.

I can barely imagine Cakewalk would only now start developing the OS-X version... I think they already have Mac developers on board for a while now.

The whole thing about Sonar Platinum was that it was to be the end of versions, to be updated continuously.  The model is similar to Windows 10 - supposedly the last version of Windows ever, to be updated indefinitely.  That's why it was not called X4.  So you can't compare the life expectancy of Platinum against previous versions like X2 or X3, etc.  The model is now completely different.

SEVerstraten
I can barely imagine Cakewalk would only now start developing the OS-X version... I think they already have Mac developers on board for a while now.

I agree with you completely - I was just looking at it as a point of accelerating to the end point. 

BRuys
The whole thing about Sonar Platinum was that it was to be the end of versions, to be updated continuously.  The model is similar to Windows 10 - supposedly the last version of Windows ever, to be updated indefinitely.  That's why it was not called X4.  So you can't compare the life expectancy of Platinum against previous versions like X2 or X3, etc.  The model is now completely different.

Someone gets it 

Bristol_Jonesey

BRuys
The whole thing about Sonar Platinum was that it was to be the end of versions, to be updated continuously.  The model is similar to Windows 10 - supposedly the last version of Windows ever, to be updated indefinitely.  That's why it was not called X4.  So you can't compare the life expectancy of Platinum against previous versions like X2 or X3, etc.  The model is now completely different.

Someone gets it 

Except that the consumer consciousness has been conditioned to expect a newer model periodically. The market, the world, all are in a constant state of neverending change. The current philosophy will only last as long as it yields sustainable results. The moment it fails to, change will happen.
 
(edit) To clarify, Cakewalk is entirely free to offer whatever they wish however they wish. It's the consumer's choice to buy in or not. My simple point is "nothing is forever".

BRuys
The whole thing about Sonar Platinum was that it was to be the end of versions, to be updated continuously.  The model is similar to Windows 10 - supposedly the last version of Windows ever, to be updated indefinitely.  That's why it was not called X4.  So you can't compare the life expectancy of Platinum against previous versions like X2 or X3, etc.  The model is now completely different.

The choice of Windows 10 is probably a bad one if you are trying describe an infinitely upgradeable last version ever model. Microsoft will end all support for Windows 10 October 14, 2025, and mainstream support (after which only security updates will be offered with no other new features) in just a little over four years. 
http://windows.microsoft.com/en-GB/windows/lifecycle
 
That is pretty clear information from Microsoft that Win 10 is not a forever product in any way. Although it was announced a week prior to the release of Windows 10, the myth of the last Windows ever, and the promise of never having to buy a new version of Windows still persists. The OP raises a legitimate concern for anyone considering an upgrade and expecting the "core" support to last forever. A date certain for an end to the support lifecycle for this version of Sonar would be useful. 
 

Oh dear, yet another company rip off! I use platinum nearly day and really love it, I look forward to the updates like a kid looks forward to' what's in the big box at Christmas. So I have paid for the updates, thinking warmly of the Sonar team beavering away making changes to the software. Now I am confronted with making a payment for updates that will continue forever although given previous comments 'forever' may not be forever! As I understand it, my payment has a discount for the updates already paid for earlier although it's time sensitive so if you can't afford it yet ... well. Not happy that I have no idea how long Platinum will be supported or the way Cakewalk seem to be stringing us along! Might be time to cut and run ... PRO TOOLS? 

Alternatively, upgrade each year for the same price. If you were happy before with this idea, what's changed now that you only have to pay it once and get updates until whenever the product is retired? You'll at least get your money's worth. I dunno if I could say the same thing about the pricing structure of ProTools.