The killer feature is ecosystem: Easily and reliably reusing other libraries and tools that work out-of-the-box with other Python code written in the last few years . There are individually neato features motivating the efforts involved in upgrading a widely-used language & engine as well, but that kind of thinking misses the forest for the trees unfortunately.
It's a bit surprising to me, in the age of AI coding, for this to be a problem. Most features seem friendly to bootstrapping with automation (ex: f-strings that support ' not just "), and it's interesting if any don't fall in that camp. The main discussion seems to still be framed by the 2024 comments, before Claude Code etc became widespread: https://github.com/orgs/pypy/discussions/5145 .
Sure you can were you should have pinned dependencies but that's a lot of overhead for a random script...
I'm not a pypy maintainer, so my only horse in this race is believing cpython folks benefit from seeing the pypy community prove Things Can Be Better. Part of that means I rather pypy live on by avoiding unforced errors.