He could've passed the torch to someone else or even sell the thing. I'm sure many people would've jumped at the chance.
Except...no, not necessarily. Being an old-style webmaster and managing servers and whatnot is a huge amount of work and requires a lot of time and energy. If you wanna run a website like E-hentai you have to be dedicated to it. Selling a website to a new webmaster would be pretty reasonable ten years ago, when independently-owned websites were commonplace, but nowadays with 90% of the content on the internet being created and hosted on like 4 or 5 sites, nobody wants to bother anymore. Believe me, I've personally witnessed the death of a 10+ year old forum site and it went down something like this. Site upkeep got too expensive and too time-consuming, so they figured it was best to close the doors.
He was the admin, yes, but he had a staff working with him developing new features for the site and optimizing it. I don't think it would be unfeasible to leave one of them in charge.
And I'm saying it's not unlikely none of them wanted to be in charge. Like I said, it's a lot of work, and the site staff didn't have to invest nearly as much time or money as the admin, so they probably had other things in their lives that would get in the way of running a website. Again, this kind if thing requires dedication. It's not some little hobby like birdwatching or building lego sets, you gotta devote a part of your life to doing it and it's gonna start to feel like a chore eventually. And given how thankless this kind of job often is, it often doesn't feel worth it.