Thanks for pointing to the Awk 5 manual (in the tar archive, I haven't seen it online still).
I've written the previous comment asking "why namespaces based" only on the directly online accessible info. Now having had the time to download and unpack the sources and read the documentation there, it sounds much better than it appeared based on the announcement.
Long term it shouldn't make gawk less compatible than it already is. Gawk already has gawk-specific extensions, and that would be one. It seems that the command-line example is the only "breaking" aspect. And even that could be theoretically modified to be non breaking, but it's hopefully seldom enough used to not affect too much.
And the "namespaces" is in fact a "module" infrastructure extension, introduced specifically to support the construction of module-like libraries (specifically solving the problem of exporting the limited number of variables from the module).
I especially like the writing of the author of Gawk manual, I've really enjoyed it even almost thirty years ago.