"All browsers work pretty well, and being slightly faster or using slightly less memory is unlikely to sway users."

Faster browsing and less memory use is the only reason I've ever heard anyone switch browsers outside of privacy issues. If Servo is significantly faster and uses less memory, and if it can keep that up for years like Chrome did, I wouldn't be at all surprised to see this flipping in a few years. Thankfully, he's no longer CTO at Mozilla so this actually has a chance of happening.