I wonder if a timed open source license would work? One in which code/features were automatically scheduled by the license to revert to GPL/BSD/whatever at a point a year or three in the future?

I think that might sap some of the impetus to reinvent those features in a competing open source project (they can of course just copy them if they wait), and also might entice people to pay that wouldn't normally because they don't want to get on the paid product train. Users also get a known date a feature will be available, a real date, not a projected delivery date.

In a lot of ways it's what many companies that develop products do already to allow people to get familiar with their product (have an open source version with less features), but this allows them a better story, makes users feel more sure about what's going on, and if they release the code immediately but it's unusable for a period by other projects, that does make it harder for those projects to cleanly reimplement unless they're sure they haven't seen in (that might be a net negative for the public with a litigation happy company).

The hard part would be tracking the time, but git has ways to make that pretty simple.