Actually, there was Qatar Foundationband Qatar Airways before Rakuten, so Barca have shirt sponsor for 10 years already. In past most Barcelona players were from academy and club didn't spend so much money on transfers and were able to live without shirt sponsor. But things have changed under Rossel - he changed club business model and started buying stars like Neymar for hindreds of millions. And as we see, such model with terrible management doesn't work and club now in huge debt.
I am actually thinking this crises will be good for Barca in the long run, maybe 5-10 years time frame. Some if not most of the iconic players in Barca have started their processional carriers from Barca youth team. To write some of them from recent history that I remember from top of my head:
Lionel Messi - he came as very short kid (some disease?), nobody believed he could play professional football, there were some treatments involved in his early days
Xavi, Andrés Iniesta - You don't get better midfield duo than this
Carles Puyol, Gerard Piqué - proper defenders and dressing room leaders, team captains
Sergio Busquets, Victor Valdés - I don't rate these two as high as before mentioned, but Busquets is still very important part of the squad
If Barca turns to their youths and it seems like it will have to....