My current playtime for the game is at 6 hours, and I don't even have the Platinum Trophy yet. The ending of this game is brilliant, and is just so damn charming. I'm not a Trophy hunter, but I could see myself perhaps going for it, simply because I want an excuse to dive back into the world and play, since I collected all the artifacts and puzzle pieces.
In my mind, I 100% see this as a full fledged game. Maybe it's because I grew up in the 80's, and more than a few of those games didn't have nearly as much content as Astro. Just because it's a pack-in game, and doesn't feature 30+ levels and a dozen different worlds, doesn't make it any less of a full game to me, at least.
We talk a lot about game bloat, and how so many games pad their time to stretch it out, but I think Astro does a great job of presenting a focused, streamlined experience, with no bloat, and instead opted for well designed levels with genuinely fun, unique, and creative gameplay in each level.
I suppose you could perhaps add boss battles to each world (much like Astrobot Rescue Mission), but outside of that, I can't say I need the game to be as long as a LittleBigPlanet or Mario Odyssey. I think this game works so well because it doesn't wear out its welcome, and it leaves you wanting more, because what's there is just so damn good and well designed.
If Team Asobi ever decided to release extra content for this game (like a 5th world, and more PlayStation collectibles, and fun ways to utilize the Dual Sense), I would gladly pay for it. With the praise that it's getting (and not to mention the Trophy hinting at more to come), I'm sure they're working on another Astro game. A new VR 1 is a no-brainer, but I think we'll get another title like this as well. I think it'd be awesome to have two Astro Series going forward: The Rescue Mission series for VR, and the Playroom series for non-VR. Astro is a charming enough character to carry two series at least.