According to the vet, Dirty's kidneys are failing. Worse, according to his blood work, they've already lost considerable functionality. He's been losing a lot of weight as a result. That said...
...not only is kidney failure apparently very common in elderly cats (I mean, he IS roughly 16), but it takes a long time for the kidneys to fully deteriorate, meaning it's not going to immediately kill him. Furthermore, because it's so common, people know how to deal with it, and cats with failing kidneys can live for years more if they're taken care of right, living out their golden years without too much issue.
As of now, the Boy is on a prescription diet meant to ease the burden on his kidneys and hopefully start cleaning his system out a bit so he can start gaining back some weight. While it won't make them start working better, it'll at least keep him going for hopefully years more. Fortunately, he seems to like it, given how quickly he NOMMED it up about an hour ago. Of course... it's expensive, but the Furry Fella is worth it.
Maybe I should start up an OnlyFans for kibble money 😄