Without customization AC would become much more like a fighting game, less like a simulator/RPG, and the allowed builds would be segmented into tiers by the fans very quickly.
At the lower-end of experience with the game, It would get significantly more 'meta' because you'd have to rely on counterpicks, maps, and tiers to determine what was fair or not. With customization, players can let stats carry them through a good portion of PvP as they learn how to pilot.
At the higher-end of experience, it would get less 'meta' because you'd have fewer options to customize against the player and situation, rather than just the map or matchup.
ACVD is at its easiest when everyone else is using the same few builds and you're adapting to them, regardless of what those builds are.
Meta is using techniques that work because of the way others think about the game, that may not make sense on paper. AC:VD lends itself to an extremely good metagame because of the damage/threshold system. Almost any part can be made viable in AC:VD because the "Best" archetypes and combinations of frames/weapons are based on what the opponent is using, not what the stats necessarily are for all the weapons.
Once enough people go that route, the archetypes change, and now suddenly "meta" means changing yet again to something else, maybe something more generalized. Especially with close communities, when everyone starts picking up 3BR heavies, the first person who starts using builds under that knowledge will receive a higher amount of success than expected.