I definitely hope it is not the last one but at the same time it does so many things well that it is hard to replace by any one single product. Just the fact alone that the official development has ended years ago and the actual game engine is still getting new features added is not just insane but also something never done before. Even in whole of game industry that is really unique. The quality of mods is also still very high and lots of new mods are being made and even started. There are lots of players online and lots of match makers and other services. And to top it off there is nothing out there to challenge it.
Even if you take long break from ac and come back it will be new and different. For sure ac still has many years in it. And not just in it but at the peak of all sims on all measurable meters. Player counts, content, hours played.
I remember playing gpl a lot and for long time after it was released. It was not because I thought gpl was so amazing. It was more about the other games being not that good or the other games had issues I did not want to deal with. Mostly physics issues. But gpl also had a problem that it was relatively stagnant game at the time. There were addons that improved some of the base game engine functionality but in the end what kept gpl afloat was new tracks and car models. Not even new cars per se. Mostly new tracks. Which gpl had tons and tons.
Finally what made me move away from gpl was live for speed. It was better all around. Except mods. But lfs still added skins, online skin downloading system and autocross track creator. Also lfs had pretty solid user experience and good online racing component. It had good car and track selection. It was good, fresh and more fun than gpl. At its peak lfs was amazing with its ranking systems and such, way before iracing. So I then played lfs a lot and for long time. The community was great and there was lots of things to do like skinning competitions in addition to actually just driving. It was progressing and it was fresh, fun and interesting.
But then lfs started to become stale. Its small amount of content started cathing up to it. Development has effectively been stopped for the last 14 years. I can keep doing the same thing for a while but what kept me playing gpl was the new tracks and other smaller things. Lfs hasn't added even those. It is weird looking back how quickly lfs died for me. But what killed it for me was its stale development and just its oldness and nothing being added to it. Then ac came. It added new things that I cared about. Good quality physics, extremely well done modding support, lots of new cars and tracks. I had occasionally played gpl while playing lfs. Gpl still had new content coming. But once I moved on from lfs it was dead. There was nothing new, nothing to see, no change.
So now I have played ac for quite some years. There is nothing better out there overall now that would made me switch like I did with gpl or lfs. Ac has just so many things going for it. Everything from gpl and lfs and a lot more. It just keeps getting better. There is also no chance of ac just dying of developer inactivity like happened with lfs. Even though official development has ended years and years ago.
So for me to move on from ac I think what it takes is that ac stops improving. That effectively means that people just lose interest. Once fewer and fewer people play ac and make mods and improvements for it and fewer people play online... once ac goes from being actively improved and new stuff being added to just a library of content that was once released then I'd guess it will be the end. It can be a slow death like with gpl if there is nothing to take its place. Or it can happen quickly like for me with lfs when something else is out there. But as long as you can take, say, a 6 month break and come back to improved game with loads of new content.. ac is not going anywhere.
The thing is though even if ac2 comes out it is not a guaranteed replacement or even an improvement. For example rfactor1 was the king of sims and modding for quite some time. But rf2 did so many things wrong that it is almost impossible to look back and think its predecessor used to dominate. Ac2 can also make many mistakes and not end up anything for anybody but a very small and vocal group of people. Maybe after 10 years we look back and think how could ac2 get it so wrong. Or we could all be playing ac2 and seeing no reason to even occasionally drive ac1.
So it can so many ways. Ac could just die on its own with nothing out there to really replace it. Or it can die relatively quickly if something better comes out. Or ac can just fizzle out as people move to the other games. Or ac can just keep going for really long time if the important key people stay interested and new people with the right abilities can step up. However, sooner or late the age of the sim starts working against it. More mundane upkeeping is required to keep it running on new systems and once that starts to become not fun. I think eventually people will move on. It is just life and life happens in many ways. I very much like ac but if we are still playing ac after 10 years I'd find that more worrying for the simracing as a whole than compliment to ac. It is good. But that good?