That's up to Squenix what they choose to offer in the cash shop. Realistically, anything that WoW offers in their cash shop is fair game in any other MMO. If WoW can get away with it, other MMOs can too.
Blizzard has not offered raid gear in the cash shop because they have always decided that it is not necessary. If in the future they deem it necessary, maybe one day they will. I suspect they won't after 5 years of WoW's cash shop not offering it.
Slippery slope arguments are astonishingly stupid anyways when WoW has had their cash shop for 5 years now and look they're at 10 million subs again. WoW has survived the cash shop, hell even 2 widely disliked expansions couldn't kill it. This game will be fine with cash shop too.