The Vuelta was very different back in Kelly's day, Jalabert was a completely changed man, far more than we've seen from Gilbert, and di Luca and Valverde showed much more stage-racing promise in their early careers than Gilbert has (di Luca 2nd in País Vasco 2000 and Tirreno-Adriatico 2002, then winning Brixia in 2004; Valverde was 3rd in the Vuelta in 2003 before he'd won any one-day race bigger than the Klasika Primavera and so was a GC man at least as long as he's been a classics man).
Gilbert might just be able to manage a top 10 here or there if there's a high attrition rate and he changes himself fundamentally, but is that really worth potentially sacrificing his other side for?