Have you responded to the wrong comment? There's nothing in the zygote model requiring file descriptors to be kept open to anything at all, and it's not racy if you keep each application in its own root (as nixos does: I'd agree this is one of the things it gets right), because the binaries and non-shared datafiles for a given package are never overwritten.
In fact with only a few exceptions (e.g. Perl if you want to use CPAN), you can mark such subtrees immutable and nothing whatsoever breaks. (I've been doing this for years.)
Yet again, this seems like a massive invasive change for very nearly undetectable benefit -- and it still doesn't solve the only instance of this problem that I can ever recall encountering, the 'updated application state in your home directory breaking downgrade' problem.
It's a shame I'll not be at LPC or we could have a good argument over this :)))