I dont know about you, but when I bought AC as a racing simulator, I was expecting a complete GT2 and GT3 class plus some track day cars and some supercars. After one year we have one GT2 and two GT3 (which are in game almost from beginning) besides that we the 599XX and the Zonda then we have a bunch of common vehicles and some "oldies" which are cool but not enough to call AC a racing simulator.
This means that for a race in GT2 or GT3 class everyone must be driving exactly the same car just because there are no cars, without several options for same class the things become really boring after some time. That's why third party mods with these cars are starting to appear, exactly because the game need these cars to survive, otherwise the game will be devoted to oblivion.
Third part mods are greatly appreciated, after all is what keeps the things alive, now when some one asks you money for something that under normal circumstances should belong to the game, is like I said, at minimum .. ridiculous.
But not everything are bad news, November is close.
You had the opportunity to see every bit of content you were promised for the 1.0 release, and are now even getting more cars than were initially advertised. I'd like to see when and where you saw that there was supposed to be a full GT2 and GT3 grid with stock content, as I've never seen it.
Kunos are a small team, and had to work with a limited budget at the start. They were able to get some HUGE names right from the very beginning, and are now getting even more companies in. That's how this entire thing was advertised from the very start; Kunos offer us base content with the purchase of the game, they use the profit to further develop the game for some years after release, and we purchase further content via DLC. Nobody can promise that AC will have a full grid of any type of car, but it is likely that it is going to fill up much more than where it is now.
I don't think you've actually raced the GT cars with other people, because to say that it's boring just makes no sense to me. I've participated in the club races here at RD, both EU and US, and it's some of the most fun I've had in sim racing. And you know what's never come across my mind? "Wow, this wheel to wheel racing that has me sweating bullets is really ruined by the fact that there's other cars around me that look similar to mine." You should try them sometime, they're quite awesome.
And that's not to say that I don't want other cars, I want an R8 Ultra more than anything else in AC right now, but I'm not going to say it's not a sim because it doesn't have one, that's silly. I have quite a bit of fun learning all I can about the McLaren, and I'll tell you what, it's a lot to take up. You should try it sometime, maybe it'll take your mind off of the other Mac's driving around you.
As for the mods. I welcome them with open arms. The URD team have done such an amazing job with the Corvette; A work of art. I plan on buying this mod once it leaves beta just because I'm sure the cars are going to be fantastic. We've done races with the URD Corvette mixed in with the official content, and you cannot tell the difference in terms of quality. Yes the physics may be a bit off, yes some data may be wrong, but I'm also sitting at my desk with a toy wheel pretending to drive a fake car. Just saying...