A boring post follows - you've been warned.
The Spanish structure is very different to the English one, a product of geography as well as history. Even on modern roads it's a 10 hour drive from Barcelona to Malaga say. Now add in the Spanish islands and go back 80 years or so to the founding of La Liga - a lot of those journeys would take a day or more, each way. A national league structure like England's wasn't practical except for the top teams.
So, Spain's got:
La Liga
Adelante - where Barca B and Villareal B play
- basically these are the two professional divisions.
Then come
2B - 4 divisions, Real and Atletico Madrid Bs, Valencia B and some other Bs are in here
They're basically a mix of Liga/Adelante B teams, and semi pro
3 - 18 divisions - more B teams and hundreds of semi pro / amateur clubs
Promotion/relegation takes place throughout, with playoffs as needed, but an A and B team can't be in the same division.
In other words below the Adelante, it's a very flat, regionalised structure with a mix of (mostly young) reserves from the pro teams and everyone else. It's not a preserve designed for the benefit of the mega-rich, it's just the structure.
The only real debate in Spain came when some teams got too strong - Real Madrid played its own reserve team in a Copa final. Reserve teams can't get promoted to the same division as their parent, and now they can't play in the Copa etc but otherwise they're just in the system like the other teams.
A retrofit in England? I can't see it, but I can see why AVB thinks it's a reasonable idea.