Hi TheSword.
I understand that you are not into Spelljammer and that's fine. What, however, makes you say that the multiverse and the notion of portals were introduced with Planescape? I was around at the Gen Con when it was announced that they were going to be releasing Planescape and TSR hosted a panel of the designers to describe it. I can assure you that we had playing in a multiplaner multiverse for many, many years before Planescape. Manual of the Planes was released a few years before 2nd edition even came out. And before Manual of the Planes was released, there was information on all of the planes in PHB and DMG and in Deities and Demigods. Manual of the Planes was exciting because it provided more detailed information on the planes, not because it dramatically altered anything (apart from all the new Elemental Para-planes and other details that are a natural result of the extra attention of an entire volume dedicated to traveling the planes, of course).
The idea of a city with many portals, Sigil, seemed a little underwhelming at the time. It hardly seemed innovative. It simply seemed like a convenient idea for a faction-based setting of plane-hopping. The idea of the Lady of Pain I did like and that did seem creative. I was down with Planescape, but as someone who lived through these changes, I would say that there is nothing from Planescape that rendered Spelljammer obsolete, which is why many of us have longed to see it return.
Cheers!