forever is not a time scale that software works on. without serious effort you cant run most programs from 15 years ago that you own outright, even if you have a floppy drive still. do you?
There's a whole secondary market for support of legacy systems
I certainly can open some files I created 15, 20 or more years ago. I've done a fair amount of IT in my career, and on several occasions I have pulled an obsolete machine out of storage, installed an obsolete OS, such as Windows 3.1, installed some ancient specialty software from a stack of floppies; all of this was to retrieve / export some important document or chart or bit of data in a proprietary format. But you're correct to say that's uncommon.
I could also envision inconveniences or time delays that might occur if an elderly/inactive photographer dies and the family is not able to get to his/her work in a timely way. Again, this is uncommon.
And another scenario: a student joins CC on the cheap (65% discount for the whole suite) while in school, and learns to use it well. Once out of school, the price becomes a substantial monthly expense to someone just entering the job market.
I think what really irks me is that, once they have enough CC addicts, they can jack up the price annually to extort just a few more dollars from each customer. If I refuse, I don't merely lose upgrade rights, I lose rights to my current software. If I'm going to commit my time and effort to learn and use a complex product, I don't want to worry about whether I'll feel the price has become extortionate. It reminds me a bit of the days before cell phone numbers were portable: if you wanted to switch to a cheaper/better plan you had to give up your cell#.