This is a pretty stupid leap of logic on your end.
Blizzard removed the unique transmog sets from the challenge modes in MoP, and the unique weapon sets from the challenge modes in WoD. Clearly that must mean they considered them flops too.
/sarcasm.
Fact: They're removing the mage tower, to give those skins an air of exclusivity. To make them bragging rights rewards and thus make the players who were able to obtain them feel more special. This doesn't mean newer expansions wont have challenges that reward cosmetic stuff. We had them in MoP, we had them in WoD, we had them in Legion, and guess what genius? we'll still have them in BfA.
Good god, some people are stupid.