This is usually true but sometimes not. For example products like Grey Goose vodka make billions of dollars despite being indistinguishable from other quality vodka even by self-professed experts.

It's probably splitting hair at this point, but Grey Goose isn't bad vodka, it's just no better than other "good" vodkas (whatever "good" means in context of a product whose defining characteristic is the absence of characteristics). If GG was a foul-tasting vodka, no amount of advertising would save it.

this ^ I didn't say you need a better product, or the best product, but you need a great product. If people hear about your product, and they think it's great, then they'll buy it. GG vodka is a great vodka (not a vodka pro but definitely in the high end of consumer vodka I believe), and coupled with their advertising they sell a lot of vodka.

That's anecdotal.

How do you define a great vodka? With blind testing?

If its like beer, then there's a few types of beer and perhaps some spice but its mostly the same.