But these aren't professional modders. They got hired to do more stuff *after* the mods. If they had demanded to be paid up front, those mods would probably not have gotten much traction. The whole point was that now that you've paid $50 for a game, some of your fellow game players have found a way to extend that further. That is, "fellow game players", not "some random guys who want money".
With Doom you had hundreds or thousands of new levels out there, people doing it for free and knowing that they would never be paid or get a job because of that, and they did it anyway. They just wanted to show off what they did, or they wanted to make something fun for other people. But they didn't have the silly attitude that time is money. In fact, Doom had a good model in itself - the first episode, a FULL game was free without the normal guilt trip whine to be paid that most crappy shareware had at the time; only the subsequent episodes were paid and you had plenty of time to do a full and thorough evaluation. Plus you could write your own free levels for this free game. Win, win, win all around.
If the great modders create a great product then they can get donations, just don't require the money to be paid. If they think they're not getting enough in donations then they should reevaluate why they're writing the mods in the first place. If they only create mods to be paid then tough luck, get a better job. But if they create mods to share and to improve the community, then the money shouldn't matter. If they want mandatory paid for mods then they should do it smart - do the Doom way, make the first part great and free, then people who love it will pay for the next parts.