@moozooh
The story has been steadily progressing in the sense that time has passed and now they are here. Obviously, things happened in that time and since no characters are exact duplicates of others, the parallels between Guts and his current 'family' are not completely identical to what he had with his 'comrades'. You could give a summary of any story and it wouldn't be complete without including all the things that got the characters from beginning to end, and you could argue that for that reason all of those events are important. However, the issue is that very little of meaningful substance to the original elements of the story has happened in that time. Like a house being built with brick walls on a strong foundation that now has a 5-car garage made of cardboard and still doesn't have a roof. All of those things make the house what it is, it has character, but I still wouldn't like to live in it and it hurts me to drive by once every 3 months and see the cardboard garage getting bigger. I understanding introducing new characters so you can expand the world and build the story in new directions, it's just that past the point where each character joined the party they got stronger but have not grown in personality or changed in a permanent way, and so they are still completely insulated from the world around them and only barely attached to the story as a whole. These side characters were introduced in their current forms 150-200 chapters ago and they've only gotten fancier weapons since then; Farneis is still a caretaker with no confidence, Schierke is still a walkie-talkie and a sensu bean, and Isidro is still a fucking monkey. Maybe that would matter if their plot armor wasn't so apparent, but as it stands the combat has also been lacking impact for some time so the importance of maintaining a character simply to maintain that aspect of the party is lost. The best arguments against this are Farneis and Shierke's shifting dynamics with Casca and Guts respectively, and like every other meaningful event those didn't happen until recently.
The points you make about the moonlight child and the new unknown relationship between Guts and Casca as the valuable parts of this arc feed into my point about the things that used to matter; all of those elements tie into the brick wall solid foundation story that was important back when characters changed and the world was being built around them. Obviously in 5 years when Guts is dead and Isidro is the main character and Schierke is his travelling companion and Casca's the newest God Hand member and the cardboard garage has simply become the new house we'll all look back on this as stellar character building because we'll be forced to assume this was the plan all along, but in the meantime I feel like the complete lack of consequence between most of the cast and the larger story is something worth talking about.