I think once a program is established and has all its main features in there and polished, the 18 month old version may not be very different than the current version for practical purposes. I.e. if this model works, it may only work well while they're getting started and adding new useful primary features.

Even in vim new features often have me check which version I'm running. I'd say that when you start using more than the basic features then updates start becoming interesting.

The other post about Github sponsorship works; it means the code remains open source and the application free to use, whilst the developers get paid for contributing to it.

That said, I think it's mainly down to companies that use the open source software to start paying for it. All companies that use open source software - and be fair, this is pretty much all software companies - should reserve some budget to sponsor and donate to open source project and developers.

Second, there's a lot of open source projects whose developers are paid for by a company, e.g. Linux, Chromium, etc. That code is still open source and free, AND the developers get paid.