If history telling us something, python only take off after 20 years and now D language is around the 20 years mark. But fortunately D has passed the growing pain similar to python 2 to 3 when it transitioned to D2.
Personally I think D just need a big breakthrough in a form of a killer application like Ruby with RoR, and Go with Kubernetes to make it more popular and gets the snow ball rolling. The killer application probably involve some kind of data science or data analytics that is currently more popular now with python. But python seems starting to reach its limits and it will be good if D can take over.
For rust since it came from Mozilla team I'd expect by now Firefox is already re-written in rust by now but it seems yet to happen, and this article probably tell us why [2]. It will be wonderful to have Firefox-rust (if it ever happened) to replace Chrome that keeps eating away my quad core CPU and 16 GB ram laptop memory once opening more than 20 tabs.
[1]http://erdani.com/hopl2020-draft.pdf
[2]https://hacks.mozilla.org/2019/02/rewriting-a-browser-compon...