Okay, let us say that you live in (flips through) Texas, and you want to go to (randomize) Kazan, Russia. Now, would a portal that takes you, in a single step, from Texas to St. Petersburg be superior than building a bi-plane that takes you through storm-wracked skies, past air pirates, and through the anti-air defenses of Europe? I say yes. Sure, it didn’t get you point to point exactly to Kazan, but even if you have to get on a plane in St. Petersburg, that plane trip is faster, safer and cheaper than the trip from Texas via plane.
So, yeah, I get that a portal isn’t going to take you exactly where you want to go, but that doesn’t mean it isn’t still the superior choice. The only point in time it would seem to be inferior would be if traveling in that world is WORSE than traveling via Spelljammer.
Considering that I saw it expressly said that there are portals from Sigil directly to other prime material planes, I find your odds of “slime to none” highly suspect.
I also don’t understand what this has to do with the Hordes of Fiends you were talking about before. Since this seems to be a non-sequitur from the discussion.
There are very few reasons why people travel great distances. The majority of them have to do with making money or conquest. And surprisingly, building magical artifacts capable of carrying multiple people through insanely hostile territory is something that generally is expensive. There is a reason most ship captains during the Age of Exploration weren’t hobbyists, they needed monetary backing to even afford a ship, and people weren’t donating them out of the goodness of their hearts.
Trade by selling what? The plants that they barely had enough of to support their population?
Long before they end up “spelljamming” they are going to explore the surface of the planet. Set up towns and build infrastructure. It may take generations before they have settled enough to even have resources worth trading to others, let alone the free time to stop building a civilization to go and see if a different planet nearby has neighbors, who could well be hostile.