I'm pretty young, so here's my borderline zoomer perspective on this: they mostly just grew up, or moved onto other things - it's already been said.
I think this may come across as unsurprising, but a lot of the "uguu so kawaii desu!" weebs that I met back in the day were literal children and very young teenagers (possibly the lot of them with autism or such other disorders as well). I was a part of this camp. There was, and still is, a lot of adults that also acted that way, but both groups moved on. The former became more self-aware as they physically grew up and became more mature, whereas the latter never really gained that self-awareness, but anime also just... stopped being niche. There was no real reason for a socially inept manchild to care about it anymore, it stopped being 'unique', a way of rejecting your surroundings, of immersing yourself in something that's new and "cool". At the time, a lot of it resonated with people who wanted to disconnect themselves from the society around them, the mystique of this dream land of Nippon, where every woman is gorgeous, everyone is polite, the economy is amazing, and due to a warped perception because of anime, the idea that you won't be judged for being inept and will make friends and connections easily.
As anime became more mainstream, and by extension more 'boring' and less unique, so did Japan by extension. Problems such as war crime denial, racism, economic issues, ideas that lend credence to the reality that Japan is not a perfect nation as one would believe from hearing about these things secondhand, from anime and from Japanophiles (the people who sourced most of the information on the nation at time; naturally, they would be biased), and so the escapist idea of a mythical island to the East where life was good, where you could 'restart', live in a society that you can adjust to, and be surrounded by pretty girls was gradually worn away. Being a weeb became ironic, an indictment of these ideas of perfection and of running away among people who liked anime, and it mostly became replaced by "god I wish my anime waifu was real" instead: the desire for a perfect reality to exist and replace the current, flawed one, as opposed to the delusion that it already does and you just have to look for it.
So the people who benefited mentally from that sort of shield moved on to something else. Unironically, a lot of them trooned out, because the sort of echochamber and community that troons form actually have a lot of aspects that appeal to the ostracized and to the desperate. But others merely went back to other, niche things, such as cartoons like Steven Universe, or things like that. The mentality that leads to one being a weeb still exists, it just requires the right environment to thrive, and the anime community simply does not provide that anymore; it needs to be found elsewhere. I'm honestly not too familiar with all that's happened afterward, simply because given that I was either a child or a teenager for most of the weeb craze I just didn't follow the same path, but I know for a fact people who would've been Ken-sama weebs back then are still around and doing stuff.