So then don't pay for it.
You imagine this fantasy alternative where these items would still exist but would be available in-game through in-game means. This is incorrect. If these items didn't represent the potential for additional revenue then they would simply not exist at all.
Then why do you pay for expansions?
Cosmetic microtransactions are extra items added above and beyond the cost of the sbuscription. That's the whole point. If the cash shop didn't exist, Blizzard would simply not create this extra content.