Its technically possible to have mods on the consoles, but there's no good way to do it as far as I'm aware. Best solution is really just having the developer handpick good user mods from the PC and then porting and integrating them on the consoles. Which isn't really modding at that point.
You don't just sell a $40 PC game for $20 on the consoles. If you want to do that, you sell a $20 PC game. A lot of people just assume lowering the price of something gets you equal or better return on investment by increasing number of sales and it doesn't work like that or most products would be priced far, far lower than they are. I'm sure the game costs $40 on the PC for a reason. Its not just your average 'indie game'.
And no, I don't think there'd be much lasting interest from most GT/Forza players. I'm sure it would get some interest, but it would be short lived. The novelty of the realistic handling would wear off and people would bore of the minimal content available and likely minimal single-player campaign. I think you overestimate how many people play these games for the heavy 'sim' aspects. Its something that probably sounds nice in their head but people love the high production values, the wealth of content and the long, fleshed out career modes where you level and unlock stuff and earn money and have that carrot-on-a-stick effect that AC simply wouldn't have.
You also bring up a good point about controller users. Controller support *isn't* great. Its passable, no doubt, but its far from ideal. I also don't think its quite as popular as you think. Yes, we've had a few people popping in here to ask about the game and how it is with a controller and people saying they'll buy it but how many of these people post even somewhat regularly? That's no proof of anything, but I think it is symptomatic of how people like the idea of a hardcore sim more than they do actually playing it for any length of time.
I think I'm somewhat an outlier in that I'm actually a hardcore racing fan but still use a controller. I do that because a decent wheel/pedal setup that I'd be happy with is expensive and I'm still a little unsure of how I'd incorporate it into my PC gaming desk/chair area. So don't think of me as some prime example, either.
And lastly, games like this are very unlike console games in that they are setup for longevity. Assetto Corsa isn't going to burst onto the racing sim scene on final release, guns blazing. It will have an impact, but its success will be measured long-term, not how many copies it sells within its first couple weeks. Its mean to be something to be honed over years, like other racing sims, not just blown through and traded in as people wait for the sequel. Is the audience really going to be there for this game in 3 years on consoles, assuming AC releases tomorrow on the Xbox One and PS4?
The market just isn't there. I would much rather them focus on the platform where the niche audience exists and improve on that. That's where people are enthusiastic about this game. Not just interested, but enthusiastic. And that's what you need when talking about a niche genre like this.
EDIT: Just to end on an unrelated positive note:
This game is officially my most played game on Steam. I've only been playing since August, but still, its beaten out Borderlands 2, Dark Souls and Fallout New Vegas, which is no small accomplishment for an Early Access game with no campaign or multiplayer!
