Gabe Newell makes clear Valve's intention to bring back paid mods

Because developing a whole game is HUGE. A mod is something a weekend-warrior can dish out. Presumably there will be more of them if they will get paid more than they would for mowing the neighbor's lawn and they will be more likely to keep it updated.

They must satisfy their customers. People want to make money off vid products.
Playing stuff like Planet Coaster show immense space to growth.

Now, it might fall on devs to allow payable mods, especially with the crowdfunded projects.

When mods come on completed products, one potential issue is minor. When they come on works in progress, half decent mod devs can anticipate the work direction to go and start monetizing what devs actually plan to add to their product. There could be a sort of race.

This could put devs in a position to buy back what they aim to implement in the first place, anytime a modder beats devs by providing faster.

Up to now, this kind of issue is solved through donations: modders accept to donate their mods.