The Transmuters were gone - they had explored, experienced and analyzed the multiverse to their complete satisfaction and then allowed themselves to die:
"What now? Where do we look for them?"
[...] He said, "We look right in front of us."
"There's nothing in front of us."
"Not now. Because it's over. We've seen it all."
"I don't understand."
[...] "The artifacts were polises. [...]
"Then why did they stop?"
"Because there was nothing more to do." [...] "They'd seen everything they wanted to see in the outside world
And Paolo does the same - he basically commits suicide, except he instead sees it as achieving completion because he's done all he wanted to do:
"It won't be death." Paolo seemed calm now, perfectly resolved. "The Transmuters didn't die; they played out every possibility within themselves. And I believe I've done the same, back in U-double-star… or maybe I'm still doing it, somewhere. But I've found what I came to find, here. There's nothing more for me. That's not death. It's completion."
Basically, it's a transhumanist version of this (source):