Xavi played both Yamal and Cubarsi because they were good enough to play for the 1st team (the phrase "if you are good enough, you are old enough" applies to them both). Did the injuries and loss of form of other players helped them both in getting those crucial initial game time? Yes, but they justifiably kept their place due to their good performances even when there were other options as well such as Inigo Martinez and Raphinha in their respective positions (even when both of them came back from injury, Cubarsi and Yamal still justifiably kept their place)
Xavi could and should have rested them both more in the 2nd half of the season. Not just Cubarsi and Yamal, but all players shouldn't be overplayed and sufficient rest needs to provided to them all. With Flick, I have no idea what his thought process is and it'll depend on the transfer for this season, but my guess is that Cubarsi and Yamal will still be starters for this upcoming season.
You aren't an "established starter" if you are playing thanks to injury and just beating bench player.
Cubarsi is atm a limited CB who managed to beat Inigo in the starting line up (thanks to the Basque being beaten by Osmihan) and Faye who is a mistake prone CB in B team. He didn't beat any starter to be called an established starter. Same argument was made about Bojan 16 years ago when he was getting ton of minutes due to injuries and lack of depth, and the moment we fixed that it was seen that his role got immediately diminished.
Lamine didn't beat Raphinha for RW, he simply showed less versatility and it made sense to make the one who can make impact on the left play there, leaving Lamine as the only valud option as RW.
If we get Nico and he doesn't flop, and everyone is healthy, then Lamine shouldn't be a starter as things stand.
And if you good enough is a bs, there is a reason ton of talented yputh players don't get promoted this early elsewhere, even unquestionably more talented players of previous generations (Messi is the prime example) because they might be goid enough, but there is a good chance you are risking their entire career for few exciting months.