Being #5 or #6 forward for Barca are terrible jobs, you're just not going to beat out the likes of Messi, Suarez, Dembele and Coutinho. These four all need to play full-time, no manager in the world would sit any of them in favor of a less-talented player like Munir. Every year, most of the bottom few players on the roster should leave, these are just high-turnover jobs. It's a real fluke when one of these guys ever becomes a regular, the way Sergi Roberto has.
We don't know exactly why Munir has been told he won't even make the bench for the rest of this season. I doubt his contract is the reason, more likely his frame of mind and/or his level of play.
I think the best backup for Suarez is Pique. But Pique would need to get a few more games off at CB to be able sub every once in a while for Suarez.