"In an ideal world, I'd suggest doing so in a way that doesn't require a CLA, such as a carefully written license exception referencing an updatable list of licensees."

That's worth remembering if possible.

"If you actually meant "commercial use", such that the default version prohibited commercial use"

The idea is it's just like open-source software except you pay for it. People can fix it, modify it, contribute to it, whatever. Might force the mods to be shared with everyone if using free license or let them not be shared if people are paying for it. The money coming in creates active development. The license keeps them from closing things back up or pulling it off the market. Trying to figure out what terms maintain the major benefits of FOSS while preventing the worst parts of proprietary model and keeping money in for development/maintenance.

The philosophy is basically "you get what you pay for or contribute to," with source and benefits that brings. Sort of a middle ground between proprietary and FOSS. Gotta be one that could work but devil is in the details.

Note: Burroughs' 1960's OS, MCP, was delivered as source to paying customers. They could modify it as they saw fit and optionally submit modifications back. Burroughs would try to integrate good submissions into later releases. It was the first or one of the earliest proprietary and OSS combinations.