It's a great move. The early development of Rust aimed to support Servo. However, it's still disappointing that the script engine uses SpiderMonkey, which is purely C++.
There are what, 5+ rust javascript engines that claim to be production-ready? Bolting one of those on in place of spider monkey seems like a reasonable future direction
I mean SpiderMonkey works, and presumably is fairly self-contained, so I can see why replacing that isn't attractive unless you believe you can make it significantly better in some way.
It's best not to try and eat the elephant in one bite, which is perhaps where this project went wrong initially. Maybe this is a symptom of learning from past mistakes rather than a flaw.