When was the last time La Masia/B team successfully produced a top level player when the team was promoted and playing in the Segunda A?
You could also ask when was the last time we produced a top level player at all, regardless of the level Barca B was playing when he was there.
As long as he gets regular playing time I don't think playing in Segunda instead of Segunda B is really having any negative effect on player's development. IF a player is good enough he'll make it if he's not he won't. Obviously this is simplified as there are other factors like injuries and overall level of competition he (a young La Masia kid) would face in the first team when/if he's promoted that ultimately decide his fate at the club.
I mean, Deulofeu was arguably one of the best players in Segunda in his second season (2012/13) there. Do you believe his career would go a different path had he played in Segunda B instead? Yet despite his dominance in Segunda he didn't make it here nor in any other big club. Why? He has/had a good enough natural abilities (specifically speed) and technical knowledge to stay here for at least a few years as a backup but he lacked a football inteligence, which is what usually makes a difference between average and good players.
The same with Adama. He has elite athletical abilities, he's good technically but he's just not "smart" enough. He's playing in a perfect team and the perfect league that suits his strengths right now but he would struggle immensely if he'd come back here and need to play a slow posssion based football against parked buses almost every week.
We should also keep in mind that every year we'll have not 5 or 10 but mostly one or two players in Barca B who could have a potential first team future and who we should do our best to help him develop his full potential (now if that will be good enough is still questionable).
This season that player was Puig. Fati is a special case because he has jumped straight from Juvenil A to the first team without ever playing an official game for Barca B and I don't think something like this will happen again soon (if ever). And while Araujo did make some first team appearances it will be next year when he'll need to fully establish himself as a first team player.
Next season our focus should be on Ilaix. If we're promoted I don't think he'll struggle in Segunda. He's ready physically (personally I believe Segunda B is a more physical league while Segunda is more technical) so this shouldn't be a problem. Technically he's on the level if not above with all Segunda players. And from what I've seen I also believe he's ready mentally to make a jump to a higher division. And if he'll struggle to adapt initially he has a good mentor/coach in Pimienta to help him.
If Pedri will stay in Barca B (which I don't think it's the best for his development) he has already proven to be good enough for Segunda and should do even better surrounded with better players.
While I like other players like Konrad, Monchu, Jandro, Hiroki and Collado I don't believe anyone of them has a longterm future at the club. They might make some occasional first team appearances but in 2-3 years they'll probably all play for some midtable teams in La Liga or other top 5 leagues. And their future won't change drastically (for better or worse) if they'll play in Segunda A instead of Segunda B next season. But hopefully I'm wrong and we can get one or two more squad players for a few years out of those players.
I know people expect/want us to produce starters or even worldclass players like Busi, Xavi or Iniesta (won't even mention Messi because we've probably get our "luck" for the next 100 years with him) but I'll take whatever we can get out of La Masia. In times when even most backup/squad players on the market cost at least 30-40m and there is no guarantee they'll actually make it it's well worth it if you can produce a few cheap homegrown players to sit on the bench for a year or two and sell them if/when you find someone better.