I really like Spelljammer. I like the crystal spheres and the Phlogiston, and Imperial Elven Federation of Planets...er I mean Armada.
It seems like a lot of people like bits and pieces of Spelljammer, but I guess to me that's barely liking it at all. I mean, sure, you can always say you like everything except giff and giant space hamsters (or whatever). But if you don't want crystal spheres you aren't playing Spelljammer--you're playing your own custom D&D in Space game. It's like saying you're playing a Forgotten Realms campaign because you transplant the Zhentarim and some ghostwise halflings to a home brew world.
I also don't really think it's accurate to look at it as merely a way of connecting other campaign settings. It can do that in the same way Planescape can, but it's still its own setting. There is plenty of lore and feel in Spelljammer to play an entire campaign without ever landing on a known campaign setting world if you don't want to.
The beauty of the 2e multiverse was that they
left that choice to you. You could hop between a bunch of worlds using the planes or the Phlogiston, or you could play in a particular world, or you could never leave space or never leave the planes. (5e seems to be taking a similar angle on how the multiverse is connected, but with it left a little bit more open to individual interpretation.)
Whenever I run a D&D game, it's run in the multiverse--and it's always been that way. If Spelljamming, or the Planes, or whatever have nothing to do with the game, then they may never make an appearance, and the players will have absolutely no idea that they are in a multiverse with giant space hamsters powering spaceships. But
I know they are out there.
And every now and then it might make a difference. For instance, I was playing in a Ravenloft campaign, where we ended up trapped in some magical oubliette that we couldn't figure out how to escape from. I think the characters puzzled out that perhaps sunlight would do the trick, but we didn't have the ability to make any. So I suggested out of character (since the party's wizard might be smart enough to know about it) that a gate to the quasi-elemental plane of radiance might due the trick. The DM knew enough about the Inner Planes (mostly from me) to run with it, and it worked. (In retrospect, I'm not sure why Ravenloft let the gate open, but I'm sure the DM had some explanation based on us not using it to leave Ravenloft.) That's why I like playing in the multiverse. There is more stuff out there than you're ever going to use, but its there if it makes a good story.
For me, if you take out either the unique elements that make Spelljammer, or its ability to connect other worlds, you've just ruined it. It's not even worth calling it the same thing at that point.
As far as getting a resistant group interested--I'd invite them to play a stand alone adventure including Spelljamming elements. Make it a really good adventure. Show how the elements of Spelljammer that they might find silly can be played straight, and how it can connect to an otherwise normal campaign setting (unless you are wanting to get them interested in a space only game). After that, they still might not like it, but at least they've had a chance to make an informed decision.