They'll keep updating, OK. But there's no backwards compatibility in the new format.

Sketch 2 can't open Sketch 3 files.

Somehow you're forced to update if you intent to keep using Sketch.

That's orthogonal to the pricing model. Same issue with pay-to-upgarade.

Perhaps they are doing something reasonable knowing that developers inevitably want to play with the new hotness and will upgrade.

I watch the phpstorm/pycharm EAP's like a hawk as they are constantly adding stuff that will make me hand over money :).