In what way is that not cosmetic? Everything 1-74 is purely cosmetic and has nothing to do with endgame.
example
2 players, start together
1 pay for miraculous arc
1 does not
1 needs 2 weeks to levelup, 1 is 2 weeks ahead (astralkeys!)
and we all know gear changes much more in balance and strenght than runes
the arcuser will also have more goldincome and can secure other advantages much earlier than the other one
it is an advantage for sure
also, calling the leveling not the "true game" is problematic too
for many players the leveling is the "true" game while astral + raids is just the endless and boring repetetive filler between patches