I'll agree with you that immersion is the primary benefit of ship interiors.
The trouble is, the stuff you're talking about - the little details like cockpits and coffee makers - are all very small in terms of overall effort. Modeling an entire ship interiors is tens of thousands of times more difficult, and that's before you even get into making the whole thing walkable! Making something look pretty is one thing, making it not trap players in corners or drop them out the floor into space or lag everyone nearby is another entirely! It's a similar line of thought as landable earthlikes. You could easily say that being able to fly down and land on earthlikes is 'the last piece of the puzzle' for Horizons, and it might even be true, but that doesn't change the fact that actually creating that content would take enormous effort, beyond the scope of just filling in the last piece of the puzzle.
If I could see any functional content that requires it, I'd be down 100%. But the game just doesn't work that way.
That's why I'd much rather see stuff like, say, boardable capital ships in conflict zones. Boardable turrets to attack enemy ships. Boardable installations, with conflict zones around them. Conflict zones inside stations and megaships. And so on.
I absolutely recognize that in order to become an integral aspect of the game, legs must be brought closer to the existing ship content, and I think that's what a lot of people are really looking for when they ask for ship interiors. That's a legitimate want, and I think it's one that should be recognized. Unfortunately, I think that most people have taken that legitimate desire and bound it to ship interiors, despite the fact that interiors are by no means the best way to achieve that desire.