i wonder if obsessive 'whale'/rich fans aside, if idols rly make that much more than porn stars lol. tho you might as well cut out the middle man and have an idol-themed brothel
It's hard to say, but I'm leaning more towards idols making more money.
Considering just how bad the otaku problem in Japan is. You have an entire generation of adults, supplemented by people from younger generations that have completely given up on the idea of getting married, having kids, and then being forced to work long excruciating hours until you potentially keel over on the job, all because that's what you're expected to do if you're the man of the house.
Because of this, these are the people that throw insane amounts of money at whatever can appeal to them the most. Whether that be games, anime, idols, and so on. Gacha games especially are designed to appeal to them, and suck up as much money as possible. Look at a game like Fate/GO which by this point has grossed over 7 billion dollars over the course of its life (and is still going strong), the majority of that coming from Japan.
As for the idol industry, I feel like that one is potentially the most profitable, but the most demanding. These are the kinds of people that will go out and buy ten copies of the same album at a time, simply if a handshake with the idol is something that can be had. I feel that the purchasing power of idol fans is stronger than porn star fans. That said, realistically speaking it's likely that the same people that go out of their way to support idols likely do the same for their favorite porn stars.