It "could" in the sense that it's of course technically possible. But it's a _ton_ of work, and it's not clear to me that it would actually be beneficial. GNU grep is doing just fine as far as I can tell at being a POSIX compatible grep.

ripgrep is split into libraries, so if someone wanted this, they could just go do it themselves. :-) It would actually be pretty easy to get something that smells like POSIX. But the details make up a very long tail, and some of those tasks are fairly significant (supporting BRE syntax, supporting POSIX-style alternations, supporting POSIX locales).