I think you are attaching too much importance to the airline - Stobart's massive investment was in the airport and that simply had to succeed. It would have been good if the Waterford route had worked, of course, but not vital. Aer Arann played a part in the attraction of EZY to SEN and thus served its purpose.
Only reason EZY moved to SEN was to give BAA the two fingers at STN because they were expensive to operate from and EZY had to drop/reduce most of the overlap routes between both airports to make it work for them.
There cost base at SEN is low and Aer Arann at SEN had zero to do with EZY coming to the airport. Carolyn McCall of EZY does is not going to jump into another airport just because a small regional carrier is there, give her some credit.
If Waterford-LTN was such a succesful route it should be straightforward for another operator to step in, shouldn't it....?
The cost base to even start the route is very high and in the current climate airlines are not going to take such risks. Then their is what was said above.