In the open Catalan, Black can hold onto the c4-pawn with b7-b5 and a7-a6. If so, then the queenside will have to be severely weakened. I think that this approach is reasonable and playable, but many other chess players state that it isn't good. Where could my reasoning have gone wrong?
It's possible to keep the pawn but at the expense of seriously weakening the queenside and fall behind in development. Keeping the pawn is certainly playable but you will need to know how not to be busted in the centre.
Keeping or giving the pawn, both possible in Catalan. Whether you like it or not depend on your personal preference.