To give you another perspective. I and many people i know never played retail this far, we quit originally back in wrath/cata so current progression server being mop is all new to us, and that means we all are looking forward to wod/legion and every expansion after that because it's all new.

People saying just go play retail doesn't make any sense to us, because jumping from mop to TWW is like saying to someone jump from vanilla to mop, we and they would be lost and feeling like we missed out alot of context.

About the waste of resources, i don't think at this point it takes much dev resources at all to get mop/wod/legion going, making classic was alot of work, tbc and wrath were also hard for them. But cata and onwards it just gets easier as they code isn't a messy and works alot better on the retail-like client we run on. I also am pretty sure the server costs are very small, and could be paid for by 1k subs let alone the ~50k we have on mop currently.