They only have to be mildly popular, so long as they're photogenic, psycho, or something else that easily gets attention. All the brands have sold numerous guitars by picking someone who wasn't a megastar but still grabbed attention. Like
@MCF suggested, someone like Nikki Monninger. They've played to huge crowds, have decent record sales, and Nikki is super cute, fun to watch when she plays, and has a "girl next door" vibe when interviewing. Her Birds aren't modified much, AFAIK, other than the cool pickguards but I suspect if Gibson or Epi sat down with her to find out what they could perfect or do different, they'd find something.
She's ten years younger than Rex, so they'd want a couple of younger players as well, but the point is, there are people they could choose to work with that they don't. And it doesn't matter if *you* like any specific music, if it's big then there's obviously a market. If they work across generations, they'll do a lot better - and so will Thunderbird players. There are bands I can't stand, but if their bassist played Thunderbirds and Gibson or Epi worked a deal, I'd happily consider the bass on its own merits.