Right now there are only 4 - 6 guys working on it.
ATS was created by 4 people.
Now they have a 10+ team working on it. Not sure how quickly and often DLC's will get released but it will be a long term plan.
Parallel they work on the improvement of ETS2 as well, but since the engines of ATS and ETS2 are almost the same it's a good investment for both games.
Of course SCS is also busy with other games/expansions. They have given some slow hints, still they have lots of plans and so far not the right pace.
EDIT:
Muzz I guess he wants to have some screen where you disconect the brake lines etc....
Never heared that suggestion before, don't really see the point of it, but everyone has it's own opinion, values and beliefs.
oh and I think the ATS project started about 2 years ago. couple of months before ETS2 got launched.