Putting a good $15 bottle on the table doesn't tell your friends "I really want to celebrate this. It's a special occasion to me". The $35 champagne that might inded be worse, does. Especially if you are the kind that often drinks the $15 ones!

And that's imho the selling point of (at least the lower end) champagne. I still do that occasionally. A $20 difference is a pretty small price for a "signal". It's not stupid just because the content is indistinguishable. I pay for the label - because the label is the effect! It's the product!

That's also why such producers need to be careful with others using their name. They are selling a name while everyone else is selling wine.