Ahh yes, the releases and time frame. I have to agree that it does look unrealistic if you consider things to come out at a pace that reflects what you have seen so far. As Daffyflyer has pointed out correctly though, what you most likely don't see (but will see once the revamp is out), is how much happened under the hood. Honestly, whenever I launch the currently public preorder version I think it's pretty simplistic, shallow, clumsy and just... not good: all in comparison to what we have now of course, not on an absolute scale.

All engine related things will indeed be much quicker to produce from here on out, and won't require much implementation work or testing, as all the fundamentals are there now. So most engines will be made in parallel with stuff that does require more implementation work and testing: the major chunks that are the car designer and the economy part.
This also means that there probably won't be updates like the V8 one with dedicated scenarios and everything after the turbo update (which will be a big one, just like the revamp is, with new features and new scenarios to play). When we move on to the car designer we'll focus on that entirely, and if there is some time to hammer out I5s or V12s in the meantime they will just "come along".
In the final phase (full game closed beta test) before release there will be lots of time for art to be created and polished. We don't need art to balance the game, and neither needs art the game to be bug free for implementation. It'll all come together latest then.
Cheers!