Your other major problem is that much of the audience for mods are kids or teens who don't have a lot of money. Keeping modding free is what keeps content generation gated to those who intend to truly intend to better the community.
As an example, I now continually get fraud warnings on my Epic account, which I keep for UDK access, because Fortnite blew up. It's really annoying :(
A TC mod for a game gives you some distinct advantages, especially if distributed through the Steam Workshop:
1. One click installation - the Steam client takes care of everything for you.
2. Community forums and news feeds already set up - you can get instant feedback from players
3. Easier to get a foothold on someone's machine - it's easier to get someone to install your game knowing that it's a TC for a game they already have (so they may have some notion about the quality of the assets and engine) as opposed to trying to convince someone to install a completely new game.
The main thing that mods give developers is a distribution network and that is invaluable if you ask me. Getting someone to install your game is the biggest hurdle.
Creating incentives for game developers to add mod tools to their games by creating the incentive for potential mod-makers to make some money off their work seems like a solid strategy for me. (I think I heard a rumor somewhere that Valve is going to bring back the "paid mods" idea again soon, possibly in a retooled form.)