I think it was recognised from early on in his career that his determination was always lacking. He played some great games for Barcelona before he joined us, yet they then sell a bright prospect like him for around £4.5mil? I think Barca always knew he'd fail. Purely guess work of course
I think, more than anything, he was always a bad fit for Redknapp's system. He simply isn't a winger in a 4-4-2 formation. He's not a Bale (under Harry), Lennon, or Townsend — he's very much an outside forward in a 4-3-3 or to a lesser extent a 4-2-3-1. I think Lamela is a good comparison to him (Lamela is much more talented, IMO), and if you stick either of those guys on the touch-line, they are wasted.
I'm Mexican by descent, so I've seen a lot of Dos Santos, and I think he'd be successful in Pochetino's system. That said, I wouldn't want to bring him in b/c I wouldn't want to block Lamela's development.
I think, for all his strengths, Redknapp was pretty rigid tactically. There are a handful of potentially very good players (GDS, The Ghetto Prince, Kranjcar and Pienaar to some extent) that Redknapp didn't really know how to use, as they didn't fit into his two wingers, two forwards, two midfielders identity. He never really played with a #10, except for Rafa, who really is more of a second striker than a pure #10. It look Redknapp a long time to feel comfortable w/ Luka down the middle, and in a lot of ways, Bale didn't really make that quantum leap, until AVB's tactical flexibility gave him that free role to move into.
In short, I'm not advocating we sign GDS, but I certainly don't see him as a failure. He got 1 in 3 for Villareal this season, and in a team that plays to his strength, he's a quality player in a decent team.