I'll tell you why I think cosmetics are bad. As someone that has played WoW and Runescape the most out of any MMORPGs throughout my life, one of the biggest motivators in playing the game was getting new cool looking gear. Now with cosmetics in games and transmogs, you no longer have to try to look cool, you can just buy your looks or you can use the looks from easy content that probably looks better than the newer content. Why does it look better than the new content? Well I'm sure one reason is because the game developers no longer have to try as hard to make new cool looking gear.

Obviously for some games this isn't the case and lootbox cosmetics aren't as big of a deal, but for games that I mentioned above, I think that is one reason so many people feel unfulfilled with mmos nowadays. Same goes with Diablo 3 IMO.

Again this is just the reason why those games have less of an appeal to me personally. Some people not even care, but the MMO community does seem to have gotten smaller than it used to be.