- Any signature sound they might have is associated with the past (rockabilly, Buffalo Springfield, etc)
Not meaning picking on you because a few people have said this, but I disagree.
You also can't ignore the lasting legacy (and now older more afluent fans who can afford Gretsches) of bands like The Cult (Duffy was in Theatre of Hate before that rocking the same Falcon), The Cramps, Rancid, and Depeche Mode to just name a few more. It's a specific tone, sure, but not niche in the sense of being limited to a few dead genres as people are implying - that's a wide range we have already. Also you can't ignore the underground; Rockabilly (and it's many offspring) are far from dead, and you can't ignore the Gretsch/Filtertron sound as the other surf guitar (see Shadowy Men from a Shadowy Planet; who granted resisted the surf label, but admitted to it later). I just recorded a band featuring a Gretsch (semi)hollowbody, kinda barngoth (countryish gothic rock).
Being a niche is often a benefit to a guitar company's sales vs you know, more Teles and (super)Strats and LPish things. The issue really comes down to the other points you, and others, made - mostly expense especially with regards to comparable alternatives. I mean putting a filtertron in Tele neck position is a huge thing - there is appeal, most of us just can't afford it.