There’s a number of reasons that “it’s just cosmetic” is bullshit.

• Developer time and effort is limited. More effort on cosmetics is less time on other things.
• “free” cosmetics will intentionally be made worse than premium ones to drive conversions.
• the game loses the ability to reward impressive feats with cosmetics, as that harms sales.
• a non-zero amount of your time is spent being driven to purchase cosmetics.