I don't think it's sustainable to force an older version of anything, except for bootstrapping (e.g. use old Python to build new Python).

Scheme would be less work for bootstrapping but more work for users (since most users won't be using Scheme for anything else). I'd rather have less work for users, which means more potential reviewers.

It's challenging to make Python code opaque, and doing so is automatically suspicious.

(As an aside, when I'm saying "Python" I'm not automatically assuming "to run Meson". Meson has some nice properties, but is not the end-all be-all of build systems, and seems far too opinionated to be a universal build system for everyone.)