I think it is wrong to say that a player like Samper has to play every week if he wants to be at a club like Barca. If he does not get loaned out, it will be all right for him to get subbed from time to time and play Copa games.
The only players that you need to play week in week out at such an age are guys like Messi, Xavi etc. who are usually better than anyone else in the team at their position (no matter if they are 20 or 25) and who would leave otherwise. But it is certainly okay for guys like Samper (who are talented of course but no Ballon d'Or level talent) to train with the best for several years, wait for their opportunity and stay on the bench during that time.
For me it should be unusual for young players like Munir, Samper, Adama to step into the 1st team and get much playing time from the get go. Samper was 19 years old last year and played in CL and Copa. He contributed to two titles, trained with legends like Xavi or Iniesta, traveled with the team to a CL game etc. If you would ask him about last season, he would tell you it was a huge step forward for him.
Munir was 19 too. He started over Neymar and/or Pedro in Lucho's first games as a Barca manager!! Those were must win games and the kid even managed to score. Munir can himself a legit Treble winner and has also managed to play for La Furia Roja. The whole season was a damn fairy tale although he did not get games in the 2nd half of the season.
Where was Thiago when he was 19/20? He was with the B Team and played some games with the big guys but not more. 3/4 years later he is bossing Bayern's midfield and the only one always playing when fit, putting guys like Götze on the bench.
Be patient.