It's actually really funny how the same people arguing against this are also against pirating the base game. Are you also saying "nobody should do a HIB because everyone would just pirate it."? At least be consistent.
While I personally probably wouldn't either buy anything (since paying for stuff online is a quite big hassle for me, regardless of cost), and I probably wouldn't charge for any of my mods either (since they're either really small, or framework mods where adoption (and the network effect) would create a chicken-and-egg problem), I'm not saying that nobody should be able to do this.
Also, to the people saying they would pay for RTB, no, you probably wouldn't. Why? I suspect tons of people (including me) would pull their add-ons from there the instant Ephi announced something like that, because they're not fine with other people (Ephi) making money from their non-commercial work, leading to an (almost) empty mod manager. Since RTB prefs would no longer be a reliable way to expose settings to people, expect that to be far less used, too. Oh, and RTBC? For it to be worth it, people have to be using it. Considering the almost 100% overlap, who would pay for that instead of just using Steam?