You're overstating some of this. Some 750 Tis actually do need an external power connector, and the high-end/enthusiast market actually doesn't care about power consumption
as long as the performance justifies it.
If Vega were smoking a 1080 Ti then you wouldn't hear any grumbling at all. It's when you get into a situation where Vega is pulling more power than a 1080 Ti and delivering performance that's barely above a 1070 in many titles that people start to get queasy about it.
PCGH just put out their new benchmark charts and the only Vega part that can even match the 1080 on average is the 64 Liquid Cooled version, which is roughly a $650 product at the moment. You're paying 1080 Ti money for a 1080 that pulls twice as much power as a 1080, which is a pretty unappealing on the whole. The only real value argument AMD has been able to make is FreeSync, but it can't really make up for that kind of performance/value deficit.
http://www.pcgameshardware.de/Grafikkarten-Grafikkarte-97980...
Another little-discussed disadvantage is that Vega is a delicate little flower, even moreso than Fiji was. Even most board partners that normally allow you to keep your warranty while using a waterblock have decided that Vega is just too delicate to have users taking the cooler off. You put a waterblock on, you lose your warranty. Many stores are not allowing returns for them either and I suspect this is a factor (along with generally immature drivers and other problems resulting in generally low user satisfaction).