Aircraft rostering is pretty complicated stuff. Otherwise, these expensive aircraft would be on the ground too much...

For example, yesterday's MAD-BOS-MAD was on a plane that did a MAD-MIA-MAD turn after a MAD-GRU-MAD turn from the day before, etc. etc.

So you can't just magically make MIA a A350 route, without matching it with other routes / schedules and having enough aircraft to handle it all. And nobody likes it when aircraft type changes back and forth on a given route, so you need 2-3 new A350s at a minimum to realisticaly assign the A350 to another route.

It will be interesting to see what new routes show up if IB doesn't retire the A330s and when a bunch of Air Europa 787s join the fleet...