From my memory, authors that write series spanning decades often end up failing to complete.
- Wheel of Time
- Game of Thrones
- HxH
- Berserk
I hope OP won’t be added to the list, but it tbh is a serious possibility.
Wheel of Time didn't get completed by the original author sure, but it was completed by Brandon Sanderson, and the consensus is the final two books that he wrote to finish off the series, are the best books in the series. Which is quite a feat honestly.
This unfortunately isn't the case for Berserk, at least what majority of it's fandom thinks anyway. Because there isn't a well distinguished Mangaka who is taking over for it's completion
GoT and HxH are in another category where, the authors, for different reasons albeit, having finished their stories despite being alive.
You say GoT is a series spanning decades like it was an epic fantasy on that proportion, but it's really not so lengthy as some of the other well known ones, Like Malazan or Wheel of Time. George could have had this series wrapped up a decade ago, but chose not to. Wheel of Time was 13 whole books in before the Author passed away.
As for Togashi, his health issues are definitely what separate him from George, however from what I've seen from the fandom, they note a stubbornness from Togashi in refusal of assistance for the series, and so you have this bitter idea of "if he accepted some help we'd be well on the ways to the finish line, despite his health issues" With the other side saying "It's his work, he can do what he want with it".
Anyway I would say Wheel of Time out of those series is fairing the best, despite it's fandom being conflicted about the story in it's entirety. It goes to show it's better to have a completed work with ups and downs, than a work with a vaunted reputation going on hiatus and keeping it's fans waiting for over a decade.
In that time, the environment changes, and so do expectations, I would argue it's less pressure on HxH since there is honestly less competition in Shounen, as compared to the fantasy novel scene. Which is why GoT has to not only come back, but also be good enough to compete with absolute bangers of series that have come out since then "does George still have it?".