I registered just to add... as a mod developer myself, I don't see why both free and premium cannot live in the same space. Modders are people who have to pay for bills to, and being able to free ourselves from a daily 9-5 job so that we can spend that time developing mods instead would be a win/win for the gamers, the game company itself, and of course the modders. Sure, you won't really have the shared spirit of todays free-for-everything community... but i'm sure between decently sized mods it will be possible to offer royalties between eachother to package mods together. Not to mention that if someone was to rip off my work now, even though it is free, I would still be pretty angry. At least if I can charge for my creations via a platform, that platform will probably also include some sort of deterrence to other's directly ripping off the code, and also provides a legal avenue to take.
Plus, if you look at the various app stores around, there are plenty of decent free things... personally I would be happy to split my mod into two branches - a paid one that goes towards supporting my food/rent, and another that is free (with some missing features) - so that everyone benefits even if they don't have any money.
Not to mention that micropayments will be easily created as well... don't tell me that someone is poor enough that they can't spare $0.50 for 10 hours or more of quality gameplay from my mod. (I mean for christs sake... a single coffee at Starbucks is like.. $6 or more? You could buy twenty mods at that price.. If you consider how much value people will get out of a well-designed mod that they only have to pay like $0.50 or $1.00 for, I think you would agree that it's a much better use of money then supporting some massive multi-national corporation trying to extort money out of you via their coffee.
Instead of looking at this issue as in "why should I pay for something that has always been free?", it's better to think of it like "Why don't I pay $1 to support someone who cares about bringing quality entertainment to my life, and who is trying to make a fair living from it?".