By building it. Which we're not, having sunk all our money into ISS. As long as ISS is there, no one's motivated to build a better-designed, dedicated stepping stone instead of a patchwork design-by-comittee political compromise that tries to be all things to all people and ends up being inadequate at all of them. It's not circular at all. As long as ISS exists and remains the focus of manned space missions, it blocks anyone from investing in a better model.

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"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
"The Mayan symbol for "book" looks a lot like a triple hamburger, but I've never seen them claiming it as proof the Mayans had Big Macs." - KaiYeves
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort