I think porting Disgaea to PC is going to prove a very good investment. With the engine working well there it should be really easy to bring the rest of the services over. For now the Steam audience remains enamour of seeing the Japanese titles finally available and for NIS it should prove a nice windfall of revenue.
I don't think it'll help as much as you think. Disgaea 2 DHD will be very simple since it runs on pretty much the same engine with some minor tweaking.
There's reasonably large divergences in engines no matter where else you go from there (La Pucelle's skill system is completely different (and I'm not even sure if NISA has the rights to that, due to the whole Mastiff thing, Phantom Brave and Makai Kindgom use free movement and very different lift and weapon mechanics, Disgaea 3 basically gutted the mechanics and rebuilt them, though D4 is very similar to 3, Disgaea D2 is in between the 1/2 & 3/4 system, and Disgaea 5 is different again).