The indirect effects are very different, some good some potentially very damaging.
- when/if we actually get some basic competition between pay-mods, quality might rise (if it becomes competition for quality) or time invested by the modder might sink, reducing quality (if it becomes competition for price, to undercut the opposition).
- the online racing community will fracture the more pay-mods are out there. This is also in stark contrast to flight sims, which don't have multiplayer where the presence of others is as important for constituting meaning like it is for us. I keep reading that servers are predominantly empty already and that will only get worse the more entry-restricted content there is. Contained league racing is a way to combat this and to be frank we already lost most of the instant-pick-up racing with strangers in the last few years.
- given that pay-mods allow people to spend more of their time on modding means that they can produce better mods in shorter time than hobbyist modders can, further frustrating them. We already suffer big problems finding new blood and this will only make it worse. Pay-mods aren't the only nor the biggest cause here, but they will probably exacerbate the problem even further.
- if pay-mods increase expectation of quality, free mods might not be able to compete within reasonable timeframes, driving existing modders out of the scene. Might be much more of an issue if the community's general attitude does not take into account the difference between for-pay modders who work several hours each day and hobbyist modders only being able to work an hour or two a day on a project.
I do not believe it is a given that free mods will always be around. We need to make sure we keep them around if we want to have them. We can do something to combat this downside by, if nothing else, being aware of how much more days hobbyists need to sacrifice just to come close to the same end result and reflecting that in the feedback we give to hobbyist modders.
I'm sure there's other effects we don't even see yet. Unforeseen consequences are hard, mostly because they're unforeseen.

We shouldn't just accept what happens with pay-mods but turn it into an actual process where we as a community handle them and the effects they have.