I think your addressing the potential issue with this shows longevity....it isn't really introducing anything new. I mean right now its okay, but I can see my interest starting to waiver a little. I can see hardcore SW fans keep watching, because that is what they do, but viewership might decrease for the overall base.
Right now the show is running on two things:
-The characters quest where he keeps running into things we have already seen before, to bring in fun nostalgia.
-Each season keeps ending like an Avengers movie, where Mando has to assemble a team to save the day.
The show remains a very expensive cartoon, and I enjoy it quite a bit, even when it inevitably misses the mark here and there.
It ends when Grogu's arc ends; it was always going to be Lone Wolf and Cub so that central relationship and quest was all they planned for, and its success paves the way for spin-offs.
Episodes are short and Grogu doesn't age at a human rate, so they have some time, but the 5 or 6 seasons alluded to by the showrunners are probably the upper limit.
As far as endings, Din either dies in service of Grogu and a greater cause, or he settles down as father/protector to the little beast after getting him to wherever he needs to be. If there even is such a place.
We may end this with a reveal of Yoda's remote and hidden homeworld, which Din could maybe adopt as his own to watch over, or perhaps even leave behind with peace of mind. Elliot said goodbye to E.T., Billy said goodbye to Gizmo, Charlie said goodbye to Bumblebee ... Din could experience growth and heal from his past, leave the cult, take off the damn bucket, and say goodbye to Grogu knowing he's safe. Maybe visit him sometimes until his brief candle of a human life flickers and dies while Grogu's still a pre-teen.
(Okay that was dark.)
Until then, he has to eliminate the people with knowledge of him that want him for bad-guy stuff. So far that's some scientists and a handful of Remnant under Gideon but I don't see that stretching 3 or 4 more seasons, so maybe we do need Thrawn and a bigger picture.
Season 1: The Client, Season 2 and 3: Moff Gideon. Now Thrawn ... if that jumped-up Smurf is as dangerous as he was in the EU, that could go 2 or even 3 seasons but we need more than just an endless chase. Could tying all of this into Mandalore somehow be the way to go? The place has a connection to Din and a compelling story that could end satisfactorily.
For everyone that loves Grogu, they need to keep their distance from the total downer of the First Order arising and ruining the galaxy again.