> reduces opportunities for user programs to corrupt kernel memory

I don't see how it could. Kernel data structures don't go on pagecache pages.

> OpenBSD's choice is arguably reasonable

At a human level, the OpenBSD people have spent way too much time coming up with rationalizations for their obsolete VM design to back down now. Whether OpenBSD's VM subsystem is good or not, their pride will force them to keep claiming that it's good, practically forever.