Really, I don't understand: do you really think that in Asmodee are so ... to think that people interested in Descent2 will buy Imperial Assault if they don't publish Descent2 any more? They are just losing a part of market that is going to buy other products from other suppliers.
The only correct reasoning (as others already said) is that they think that the market they lose is less than the effort they have to put in developing new Descent2 contents, but this is contradicted by the fact that they always claimed that the line had positive numbers.
So, IMHO, the only correct hope is that they soon realise that they need to continue to publish Descentx physical contents also in a reduced way (1 expansion per year?). If Descentx will be Descent3, everybody of us will take his decision (to buy or not to buy?) :-)