The 757 is a much more versatile aircraft than the 321. As well as being capable of being a workhorse it has a much longer range ( I think an extra 1000nm or so) and is 180 mins ETOPS certified. The winglet fitted versions are to be seen all over the UK (and Dublin, too) doing T/A on leaner routes in these hard times.
Hindsight is a wonderful thing but you might question if a small airline like EI was wise to put all their eggs in one basket. What are they going to do with all those 330's? Whilst the good times may come back over the next ten years what happens in the meantime?
As a matter of interest how many 330's do they actually have?