The car analogy is flawed.
It's more like ... public transportation.
See, you have put your money on some equipment. but now you need to support it. You have to go to a store which is an authorized service center, and for that you have to use a third-party transportation service. You have to pay those people to take you from A to B, since it costs them to run their business.
Under the scope of public transportation analogy, suddenly it does not seem totally unreasonable anymore.
Steam does *a lot* for end users, and also for CO/PDX. It is an awesome delivery medium. It stacks a large audience, in a safe environment, and lowers the barrier for people to get in touch with each other and with the good they want (the games).
Managing all this has a cost, and there are so many people together that some bad apples came in too. And those bad apples are trying to exploit the others, and now Steam also has to act as a police, trying to catch the naughties and separate them from the goodies. This policing also increases the operating costs.
Steam is a vehicle, a distribution medium, which does the heavy lifting for us. "I want to ride my own car, I want to ride a bike, I want to walk, but I don't want no stinking public transportation from Steam". Fine, that would is the equivalent of doing all the workshop magic outside Steam. Create a forum, gather a community around, distribute the mods there. Simtropolis exists for this reason.
The barrier to mod installation is higher, the popularity is lower, it is harder to build trust but hey! Now you are riding your car.
Mods can be sideloaded into the game, it's totally viable actually.
The Steam restrictions are deterrents against the bad apples in the community. They are necessary, even if you don't like them, like paying taxes to have police. It may not be perfect with them, but without them many things taken for granted would just collapse.
The day you finally find a single other title you want to play available on Steam, this problem will vanish like a water drop in a hot pan, and all these sweat and tears are going to become a distant past.
I'd advise taking advantage of the Summer Sale right now, get something at a discount, and call it a day.