The risk with such rewrites is ending up with a Python 3 situation and an ecosystem split. Sounds like YARA-X is (mostly) a stricter subset of YARA, and it's easy to write rules that are valid for both:
> At VirusTotal, we have been running YARA-X alongside YARA for a while, scanning millions of files with tens of thousands of rules, and addressing discrepancies between the two.
This is pretty encouraging as far as compatibility. I hope they keep doing this.
https://virustotal.github.io/yara-x/docs/writing_rules/diffe...
Although I wonder how long it'll stay that way? It'll be very tempting to add new features to YARA-X that won't be backported to YARA.