What was your justification for the monthly fee in the first place?
There is a model that worked for decades: If you spent a _significant_ amount of work enhancing an existing tool you'd release a new major version. The would be a discount for license holders of the old version. Why reinvent the world over and over again?
Because what you end up with is a long tail of revenue that doesn't justify working on the app.
Simple answer right? It makes more money.
Not saying that was OPs motivation but that's obviously why the shift happened.