In an ideal world with unlimited budgets, an all-Stadler fleet would've been a game-changer for the entirety of Wales as much as it has been in Anglia.
Unfortunately that's not the world we live in, there is a limit to how much money is available. This applies to costs upfront (capital expense, CapEx) as well as ongoing (operational expense, OpEx).
If an all-Stadler fleet was prioritised over anything else, there probably would not have been sufficient budget to make it possible to replace the entire fleet.