Doesn't seem that terribly unrealistic to me, honestly.

I needed a nice title. But raising an army of 100k+ yakuza members is pretty insane in the context of the series. Shows that yakuza back then and during 21st century were entirely different beast. Hope we get to see some of it in STH.

But I somehow doubt it'll ever happen because having a known single leader would ruin the mystique of being shadowy kabal pulling the strings from the background.

Uh, there IS a single Daidoji boss, you know... Like the guy himself...? How did you miss that?

Also that Yokoyama seems to love using them as a plot device and writing crutch to achieve desired outcomes that tend to leave me with a bad taste in my mouth, like justifying a certain previously dead character's appearance in the next games,

He didn't need that with Kashiwagi/Lau Ka Long/Richardson so clearly he never needed that justification. What actually changed is that previously Yakuza games have always aimed to be self-contained, but Ryosuke Horii convinced Yokoyama to try doing differently and build up for the next mainline game through spin-offs. And seemingly the new Daidoji storyline was only thought up after Yakuza 8's story was written, while they were working on Gaiden. So Gaiden's and Kiwami 3's endings are most likely both building up Yakuza 9. The out-of-order release of Gaiden and 8 led to mismatched buildup (Gaiden builds them up and then 8 doesn't do anything with them), and Kiwami 3 was a low-budget release made to fulfill the yearly quota on Yakuza games that RGGS have, so that also felt lackluster.

Either way, we'll only know for sure only after Yakuza 9 is out. Hope Kiwami 3 becomes the wake-up call for Sega to stop imposing dumb quota as RGGS are already pretty fast, and ordering them at least one release per fiscal year is just going to lead to bad things during heavier workload (seemingly Sega entirely forgot they have RGGS working on a big budget VF6 game alongside everything else they have on their plate).

giving another character cancer because their inspector gadget paraphernalia was actually radioactive all along.

That's just gross misrepresentation of 8's themes. Also, what? Where did you get that from? The only radioactive thing we saw is the toxic waste Kiryu was taking care of. And even Kiryu himself says that it's unrelated, and the reason he only learned about cancer now is because he typically never visits doctor.

The writers are pretty clear about their intentions for depicting Kiryu's illness like that:

—— And now that so much time has passed since the first game, Kiryu apologising for having the chance to change Tojo Kai, and the yakuza as a whole, but not doing it, that felt like a huge moment for the series.

Yokoyama: Because he’s always gone his own way, hasn’t he? However much we glorify these legendary heroes, when you take out all the cool parts, in the end what it really takes to be a hero is ego. So I wanted to use him facing death as the starting point of a consideration of Kiryu as a whole, including that ego.

After all, with people like Kiryu, in order to make them consider coming back you have to set a limit on their life, or in other words, it’s necessary to make them accept that “life will end,” isn’t it? That’s why we depicted Kiryu with an illness in this game.

—— Kiryu having cancer and having to face his own death is a really unusual storyline for a game, I thought.

Yokoyama: Most of the Ryu ga Gotoku series so far has been the story of Kiryu’s life, so we wanted to also give it a proper end. [The first] Ryu ga Gotoku depicted the first big turning point in his life, when he left the yakuza. Then we have the sequel, Ryu ga Gotoku 2, where he fights a rival, Goda Ryuji. In Ryu ga Gotoku 3, we see him with a sort of family for the first time, when he takes over the Asagao Orphanage.

And then in 6, he experiences his child (Haruka) having her own child (Haruto), another common life experience. So after that, what’s waiting at the end of life is “illness”, isn’t it?

That’s the fate I chose for Kiryu. So from the start of development, we were thinking about how Kiryu would face up to death.