The possible WC win doesn't matter here. Apart from pure natural ability, both Xavi and Iniesta had the la masia upbringing, which I think counts alot. Furthermore each of them had something special. Xavi had the ability to read the game and find/open spaces like no other player. Iniesta was a big game player, winning man of the match in several big finals.
Modric is an exceptional player, but Xavi and Iniesta are legendary.
PS: and as some people here pointed out, Croatia had a pretty easy run to the final. Maybe second to what England had. But I guess it is easier to get on the underdog bandwagon...