He would barely play as a 5th midfielder and any value would drop further.
At his age it would be best for him personally to get minutes on pitch and for club to try and increase value if deemed not quite good enough.
Of course he can earn his place as a rotation in midfield but for that he has to start showing a lot more than has at Betis so far.
A problem is if we'll sell Vidal or Raki we'll only have four midfielders next season as I don't see us buying a new midfielder (which is a mistake but we just won't have money for a quality one after buying Lautaro and I'd prefer to avoid Pjanić or Ndombele). If Semedo is sold as it looks like then Roberto will be a starting RB so we can't count on him to play in midfield.
Then it's up to the club to decide if we'll count on Alena, Riqui (who is more unproven and has more (physical) limitations than Alena), Collado (also unproven) or even Pedri as squad/bench players. Personally I think Pedri is by far the most talented out of them but he does need to play regularly every week so keeping him as a rotational midfielder is probably not ideal unless we can guarantee him let's say 2.000+ minutes. But loaning him to Sociedad/Betis it's probably better for his longterm development at this point.
So do we keep Riqui in Barca B (where he won't develop anymore) and call him up when needed or will we finally convince him to go on loan to see if he can perform on La Liga level? Do we keep Collado instead because of his versatility (can also play as a winger though we should have a solid depth there with Griez, Fati, Braithwaite and Trincao basically all competing for one spot because Messi always plays)? Or is it better to just give Alena a full season to see what he can do as a bench player and then sell him next year for whatever we can get (or in an unlikely case he'll prove to be good enough to stay here longterm renew his contract).
If the best offer we'll get for Alena this summer is around 10m I think we can probably sell him for the same fee in 2021 even with just a year left on his contract. If we'll only receive loan (with a relatively low non-obligatory buy option) offers then I also prefer to keep him. But if we can sell him for 20m+ then do that and give a chance to one of the other players mentioned before. The one thing I can't see happening is Alena going on loan back to Betis (or another club) and explode there so we could sell him for 30m or so with an expiring contract. This would be ideal but I just don't see that kind of potential in him.