One angle I'm not seeing talked about much is why it's assumed it's a done deal that Luke has Grogu now. Who's to say that he's going to be successful in training the kid at all? I keep thinking about Ahsoka and the contrast between the two. Ahsoka sits with the child, "speaks" with him, gets to know him and understand him, and tests him and comes to the conclusion that he's too attached to Din and that she can't train him because she thinks that attachment could be a problem. Maybe that's just her oldschool jedi indoctrination at work or maybe she just has enough foresight to recognize how things might go if she separates them. Compare it with Luke, who pretty much shows up, convinces both parties, who are clearly hesitant, that he knows best, and takes the kid without a second thought. He's force sensitive, so he needs to be trained. Maybe Grogu's training won't go very well at all. Maybe his emotional connection to Din will be a problem ultimately. Maybe he will eventually abandon Luke and his training after sensing Din is going to be in danger, mirroring Luke ditching Yoda in ESB.
Din sent Grogu with Luke because he thought that's what he is supposed to do and what was best for the kid. Grogu went cuz he thought that's what Din wanted him to do and wanted to make him happy. I've often held the thought that the overarching plot here isn't that Grogu needs to be returned to his people, the jedi or whatever. That's just a misdirection. Din and Grogu eventually discovering that they are each others people and that they need each other, and this is just another bump in the road for their relationship, and the eventual discovery that Din is basically his Dad and Grogu his son. At least that's my hope. I want a tiny mando helmet wearing Grogu who decides to follow the way of the Mandalorian.
This could also be a first glimpse at Luke's setup for where he ends up in the sequels. Watch the scene again with grizzled Luke in mind. He's in full on Luke Skywalker, Jedi Legend mode here. The way he's mirroring Vaders Rogue One scene. His final move on the last Darktrooper, force crushing it rather than just ending it simply. It's cool and badass and everything and it's really fun to watch, but it's not very jedi like. It comes off as a little arrogant and show offy for a jedi. Think of Yoda's words and teachings. Adventure, excitement. A jedi caves not these things. A jedi uses the force for knowledge and defense. Never attack. Perhaps Grogu will be one of Luke's first lessons that being a jedi master isn't as simple as swinging a laser sword and being all mystical and legendary, eventually culminating in his failure with Ben Solo and the rise of Kylo Ren. I'm likely looking too much into it, but there may be more nuance to that ending beyond just "wow, Luke is a badass jedi."