Modern CPUs support large pages that cut off some part of the page walk. Linux supports them transparently, so if you set up a bunch of threads sharing a single large address space it will be compacted into a handful of large pages.

Single address space OS's in a broader sense have been made unviable by Spectre vulnerabilities; these days you need an address space flush at every transition from a "more private" to a "less private" information domain, which is basically every context switch when applications can't trust one another and have mutually private information that they're trying not to disclose to other apps.