I would rather see Spidey facing off alone against the Sinister Six than have them waste time rationalizing a reason for Webhead to get involved with the Thanos fight.
Actually, historically, has Spidey ever gotten personally involved in the cosmic stuff without it being Secret Wars, either of them?
Infinity Gauntlet, Infinity War, Infinity Crusade, Infinity Abyss, The End, and he was Captain Universe for a bit.