The whole point of Assetto Corsa was that it was supposed to be a sandbox that could be built and improved upon. If this announcement is a sequel, then to me, that seems like Kunos is giving up on that idea. Sure the engine has limitations, but that's what I'm hoping has been worked on.
The reason I mentioned GTR3 is that, if we're being forced to change to a new game to keep up to date, then it might as well be sooner rather than later. The little that we've seen of GTR3 looks great and it is supposed to be released this year, so why wait until Kunos have released AC2, we've gone through all of the early access again (and have suffered through all of the bugs again), have built up our collections of cars and tracks again, only to then have Kunos announce AC3?
The fact is, we don't know what Kunos are going to announce. For all we know, their next game may not feature modding ability next time around, but instead they just give us a dozen tracks, a handful of cars and the ability to buy DLC once every six months. Obviously I'm not saying that WILL be the case, but we won't know until they make the announcement. For all we know, it could be something relevant to the console releases that won't even affect us PC owners!