What is to stop ACE from getting a license and becoming DLC? Or I44? or PR. How far outside the abilities of modding do you have to be to warrant license?
The real question is how far beyond the point where they should warrant licence have mods evolved without becoming payware ? Some of the modders produce professional quality addons that take a lot of time and effort to produce and that would deserve to be released as payware. As games evolve, modding becomes a lot more complex and time consuming task and require ever more advanced skills. At some point people won't be able to do it for free anymore.
IIRC BIS gives free licence to modders to develop any non-commercial mod they want regarding any aspect of the games, and provide the required tools, but in return, anything produced by the mods is owned by BIS, which means if a modding team wanted to sell their work, they'd have to purchase a licence from BIS to do so. When you're shooting at such a complete conversion, you might as well buy a licence to use the engine, and I wouldn't be surprised if that was the case with this new game.