I don’t necessarily blame them but think their decision was pushed along by the need for big money.
For example, I think they’d still be able to do the pay once model if they abstracted they storage to work with Dropbox/icloud/OneDrive/whatever.
There’s really no value add as a user for a monthly fee. Although lots of people don’t mind. I’d rather not pay for something as essential and simple as a synchronized, encrypted data blob. I literally replaced it with a Google doc and cutting and pasting more. A filter over Google docs does not require a monthly fee.
I have this problem with lots of SaaS products that could be software if they didn’t want or need lots of money.