besides just immersion, ship interiors is the whole foundation for EVA in space, and for any activity that might include ships other than your own. You cannot exit your ship in space to enter some stranded / wrecked / damaged ship without ship interiors, so no salvage / rescue / investigation / boarding / exploration or pretty much any activity that might involve ships, either your own or NPC ships. It also means there won't be any possibility of boarding friend / NPC ships on land, either to recover items or salvage from a crashed ship, or just joining some mates for exploration or do a mission on their own ship, or do any kind of mission that requires entering an NPC ship. All of that requires ship interiors (for your own and others).
To be honest I find it just plain amazing how can any scifi enthusiast on the planet not think of anything at all that can go on inside a spaceship.

I can underrstand that some people just want to fly spaceships and that's perfectly OK, but at least they could simply say that and just stop pretending that ship interiors can have no other purpose other than "wasting time". If anyone was truly concerned about "time wasters" then they would certainly wouldn't be playing ED of all games...
Personally, I'm not against ship interiors per se, far from it - I'm just sceptical that Frontier can deliver engaging gameplay loops in this area besides looking at stuff and maybe basic corridor shooter mechanics, purely judging based on their track record.
So with that in mind I'd rather had them focusing on fleshing out the "outside" world first (planet side activities, gas giants, ELWs/WWs), before moving on to building interiors for all ships... and if they'd go down the road of interiors my expectation would be (quite rightly so I believe) that this would apply to all ships including small ones like the Sidewinder, Hauler and Adder, which, due to their small size, probably don't lend themselves well for ship interior related gameplay loops, besides leaving or entering the ship. And that, quite frankly, will likely get boring after doing it a couple of times (imo).
If they had the ability to develop these things in parallel then I wouldn't have a problem with interiors at all, but the fact that they don't feature as part of a significant, paid DLC which introduces spacelegs (i.e. the pre-requisite of making use of ship interiors) tells another story unfortunately.
So just to be clear from my perspective, it's not that I
don't want ship interiors, I just believe (and I know this is quite a pessimistic view) they'll likely suck if they were added based on other existing aspects of the game, many of which are
still placeholders 5/6 years in. So rather than adding another placeholder to the lot, better to flesh out what we have already, and I think/hope that's precisely what they're doing with Odyssey - hard to tell from the first couple of trailers, I'll reserve judgement on that until we've seen all the other dev diary videos.