Sandboxing isn't a substitute for having your drivers be open source. Even if all drivers were written in safe languages, you'd still have CVEs and still want to fix those CVEs.
Closed source Linux drivers exist. Open source Windows drivers exist. An unstable kernel ABI is neither necessary nor sufficient a condition for open sourcing drivers.
The key new "feature" of Fuschia is that its stable ABI makes closed-source drivers easier to write in the short term, relative to Linux. As a result, the vast majority of Fuschia drivers will be closed-source. Don't you agree?
I believe supporting a stable API and closed-source drivers will result in a technically inferior, more complex, less stable kernel. The Linux model of including all the drivers in a single open source codebase, where the drivers can be refactored and improved along with the rest of the kernel, is a genuinely superior way to develop a kernel, which produces a genuinely technically superior product. I just hope Google realizes this before they sink too many millions into Fuschia.
How exactly are you going to sandbox your drivers? Run them in user-mode, perhaps? But then you've constructed a microkernel, and no-one uses your system because it's slow.
Anyway, what I said is that sandboxing isn't a substitute for open-sourcing. They are complements, not substitutes.