> a stepping stone to being able to live in space long term in very large, permanently spaceborne crafts.
That is not going to happen, without technology that currently only exists in Science Fiction, like artificial gravity, for the simple reason that we require 1g to live, let alone thrive.
> because of immediate access to materials that’d need to be launched into orbit otherwise.
1. How does this "immediate access" benefit the aforementioned "very large, permanently spaceborne crafts", which apparently won't be moored to planetary bodies?
2. There is no "immediate access". Having rocks next to me, and having the sort of highly refined materials that go into building the tech required for spacecraft, are 2 VERY different things. But, I am always happy to be proven wrong: Let's take a very simple task, like ISRU'ing LOX & Methane, and let's do it, at scale, here on Earth, where there is no lack of energy, breathable atmosphere, building materials and labour. Strange, isn't it, that no one seems to be doing that.
> In a universe where there are large human civilizations not just throughout the solar system but also scattered amongst other star systems, there are numerous paths and discoveries that each branch will take that Earth’s branch in its lonesome may never have trodden.
I agree. But given that, what evidence supports the idea that the branch that eventually allows us to leave our solar system requires us to first waste tons of resources on trying to send people to inhospitable, irradiated rocks for no good reason?
Especially since we have a perfectly good alternative to this waste of time: Sending robots.
> It also just seems a bit cruel to be able to see the vastness of the universe and never be able to touch any of it, in person.
Unless we discover a way to do FTL travel, it doesn't matter if that feels cruel or not, it is reality.
And I can pretty much guarantee that the person discovering the means to cheat physics in such a way won't be doing so while constantly worrying about his habitats airlock malfunctioning, or the piss-regeneration system giving out, or the supply ship getting canceled in the next congressional-bickering about the budget.
It will happen here on Earth, likely by someone who never visited even LEO, someone who works and lives in a stable environment with books, people to talk to, air to breathe and delicious non-freeze dried food to eat, who never has to worry whether there will be enough recycled piss to make his next cup of coffee.