I wrote the linked post and maintain the Firefox build system. The reason is that in 2018 (and for the past 10 years honestly) and it is far easier to find people who know Python than Perl. Python is essentially the lingua franca in Firefox land for systems-level tasks that don't warrant a compiled language. As I said in the post, Rust will likely infringe on Python over time due to performance, lower defect rate, long-term maintenance advantages, etc.
Can't wait for a static, dependency-free hg, but sadly they aren't going to rewrite it entirely in Rust, just the speed sensitive parts. I don't care if it's written in Rust, D, Go or even C. When we picked fossil over hg and git, the Python dependency made us shy away from hg, although the CLI and the overall experience was better than git. Now we also fossil as a deployment tool; it can be a statically compiled drop anywhere binary.
I would say that there are no immidiate plans. But as libraries get portet to rust it might just make sense to start writing more features in mostly rust.
Basically I have some hope that eventaully rust will take over, however writing the core components is a critical first step and selling it for the immediate benifits is much easier then selling a complete rewrite.
