The Campaign Setting has this to say on portals:
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The book doesn't even try to pretend that portals are some subject to the universe, that there's some fiction to it that the DM then has to interpret to find out if their desire fits with the RAW or if it involves extensive homebrewing. it straight out says that the DM is in charge. And then, a few paragraphs later, it talks about the three types of portals and
how they relate to adventures. Again, no fiction here. It goes directly from worldbuilding to adventure use. For instance:
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And note that this
is specifically about adventures.
Portals work exactly like Chaosmancer says
and exactly like you say
and exactly like I say. Neither of those quotes are sidebars, either; they're right there in the main text. Which means that if you want there to be portals that allow for consistent, reliable trade between Sigil and Point B for centuries,
go right ahead. The book is telling you it's OK to do that. This isn't homebrewing. This is following the rules of the book.
And note this section from
In The Cage:
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This one is more fiction-based, but it clearly provides support for the idea that temporary portals can be used however the DM wishes, even if it's for an "obvious" purpose, like setting up trade.
Anyway, more
In The Cage bits:
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If it ain't in the Guildhall and Market Wards, it ain't anywhere. Yes, the rest of that second quote says that most of the portals are temporary, but that still allows for as many portals as the DM wishes--and if the DM wants there to be a long-lasting portal from Sigil to Point B there, then all they have to do is decide who controls it and how much gold they charge for access.