The problem is that it then gives the developer weird incentives. If someone in the community decides to build the same feature that the main developer kept as a paid feature, then the main developer won't want to merge it into their version.

Then you're stuck: the community member could certainly fork the project with their new feature, or maintain a patchset and forward-port to each new version of the original software, but both of those things are a ton of work.

Meanwhile, users are the ones who lose out; someone decided to build the feature and give it away for free, but users have to jump through hoops to use it.

As someone who has done a lot of open source work over the past ~20 years, I don't think anyone is entitled to find a business model in there. Sure, if you can find a way to work on your project full time but still support yourself, that's great. But often the ways people do so make for some (IMO) bad trade offs and perverse incentives.

I like the dual-licensing approach, where commercial users need to pay, but that's not workable for all projects.