Speaking for myself, I've never had a problem with integrating Planescape and Spelljammer - they work just fine together, and there's plenty of room for overlap - but the main issue I have with the calls for a "Planejammer" combined setting is that it would inevitably be imbalanced and heavily skew toward one or the other.Yeah, I think that it's a lot easier and less damaging to integrate the two than one would think.
One other thing that comes up in Planescape is that the Ethereal plane is the connective tissue between all matter, whereas the Astral plane is the connective tissue between all mind.
As someone who is admittedly far more of a Planescape fan than a Spelljammer one, I'm of the mindset to treat Spelljammer as a subset of Planescape - Spelljammer deals with the prime material plane, which is one small facet of the broader cosmology dealt with by Planescape. I have no issues with letting Spelljammers sail the Astral and Ethereal Planes in addition to the Phlogiston, or possibly even jump between planes directly, but even that only makes them one viable means of traversing the planes out of many - I would expect Yggdrasil the World Ash, the Infinite Staircase, the Gate-Towns, the Rivers Styx and Oceanus, etc. to get just as much focus and support.
That is how I look at it and what I would want out of a combined "Planejammer" setting, but I feel safe in assuming that pretty much all Spelljammer fans would consider that too heavily weighted toward Planescape.
By contrast, most of the proposals put forward by Spelljammer fans seem to my eyes as a push to break up the Great Wheel into a rehashed version of 4e's "Astral Sea" and drop Sigil in to serve as a kind of "space port". To put it a less flattering way, it's pillaging Sigil and a bunch of planar locations from Planescape, reworking them to fit a Spelljammer-centric model, and then leaving whatever is left of Planescape's unique identity to rot.
You'll have to forgive me if I, as a Planescape fan, don't find that a particularly compelling argument for the future of a setting that I love - it feels like someone arguing that everything interesting about the Forgotten Realms setting should be moved to the area around the Sea of Fallen Stars, because that's the only region they think matters.
Fundamentally, I don't think a combined "Planejammer" can be done in a way that satisfies fans of both Spelljammer and Planescape, and so I think they should be separate books.