Sure, a base and a fodder army is the most useless thing for a yonko. Just look at who is probably the strongest yonko currently: Shanks. He has no base, he wanders here and there with his ship, just like Luffy, and his grand fleet is actually not an help but a burden he has to protect, guess for Luffy could be the same if actually logic applied to the manga (why wouldn't marines or BB pirates or whoever has beef with the SHs go and attack the guys who joined in Dressrosa).
Also from the story point of view it would be a burden if he has now to make an island his HQ, he has to leave people there to protect it, actually somebody must eventually attack it... Now Oda is going full on the "find the one piece" route and also actually, he will not change the formula he always used and made OP what it is now.