Hero purchases are not cosmetic, neither is the grind to level 30 so you can use full rune pages (basicly minor buffs to a collection of stats). It is inaccurate to portray these things as cosmetic.

And yes, I'm sure you can 'have fun anyways' and a host of other standard pay to win excuses, but the fact remains its an element you pay for that makes you more competitive, and not simply cosmetic.