With regard to the BAe ATP and the IL-96 etc., yes, the production will end and we will not be able to order any new a/c of this type. The BAe ATP will not be replaced by BAe with any other model. In real life, it was no success, as competition was quite strong from other manufacturers (DHC-8, Saab, ATR). The IL-96 was no success as well, but there was a replacement, mostly used as freighters I think. The CRJ-100 is being replaced by the 200-series, which can be seen in the game already.
In the game availability may matter more than efficiency at the early stages. As long as there are slots available, go for them. Once the money starts pouring in, it's nice to have, but you can't make investments because of very long waiting lists for the particular planes or slot bottlenecks.