Thing is: it isn't; however much sense it might make to you.
It's really quite absurd. The idea that I buy a pair of scissors, and then the manufacturer turns up on the doorstep a year later and demands a "subscription fee" because I'm still using the scissors and they have – in the mind of the manufacturer – become a service to me is stark raving bonkers.
I buy the thing. It works. I should not then have to pay again to use the thing that I bought that works. It should keep working. What developers are doing is removing the working product for a money-grab – and for that I blame Apple; they should not permit this.
Now, if the world around the thing I've bought changes – 32-bit app no longer works on 64-bit OS, etc. – then sure, there's a cost to pay. But that isn't what's happening. What happening is that developers are taking the one-time payment, and then later adding a pay-to-play. I genuinely have nothing for contempt for this behaviour – it's theft, imo – and I will never buy another product form a company that behaves in this manner.
The only part you left out was that SOME software uses proprietary formats for the output/storage. Should one stop the subscription, SOME purveyors of this racket don't allow you to access YOUR data.