It doesn't make sense. First of all, there will be cracked versions. Trust me, just check thepirate.se within a few weeks and we'll see tons of them, some perhaps having viruses that present risks to those buying them. It happens with every other game/mod out there, and with mods selling for as much as $50 it'll be even more lucrative to download illegally. I think anyone would rather get a mod for free and then actually play on it before paying for it. If I thought that the mod was good, I'd consider donating to a patreon or something that gives cool benefits like seeing how the mod is made and stuff like that. It works perfectly for tf2. I'm going to guess there's only a half dozen users on this site that haven't dropped at least five or ten bucks on a hat or strange. And that's how they give back to the developers. However, it's better for modders than it is for the tf2 devs (in this example) because the modders get DIRECTLY supported through donations.
Valve has a lot of problems. Their customer support is abysmal, and they need to focus on that, besides a modding community that didn't need to be revamped