While nothing of what you said is wrong by any mean, we have to also consider which La Masia/Barca B player has succeeded at all since Pedro getting promoted in 2009/2010?
9 seasons and the only name is S.Roberto. Who needed 3 years at Barca bench, changing positions and return of his former Barca B coach who believed in him.
IIRC, we started the loaning thing with Alberto Botia. So also in this past decade too.
What I am trying to say, is loan at fault or it is just La Masia general decline?
I think loans are generally great if we really trust our sporting department in picking right team, that is fairly difficult condition at the moment
An argument could be made that Roberto staying with the club until he got a coach here who believed in him is further proof that these loans aren't necessary. Producing players of Roberto's quality shouldn't be hard, the guy really isn't special in any way. Even among the talents we've gotten rid of who ended up not being good enough, we could have easily filled most of our rotational options with them. The likes of Grimaldo, Tarin, Masip, Bartra, Thiago, Montoya, Cucu, and even Samper before the injuries were all good enough to fill that rotation/bench role that Roberto filled but for some reason most people are only capable of seeing a La Masia graduate as a success when they're a full blown starter. In some of those names listed (Grimaldo, Thiago) we're looking at players who potentially would be starting for us, anyways, but didn't have the support required to break into the team.
I just can't see any benefit to loaning these kids away. The manager of the new club can bench them for any reason and suddenly an entire season of developing as a player is gone. I'd rather take the risk of hoping Valverde gives them a few games across an entire season, than send them away to rot on the bench.