I've heard the same argument stated as 'must be 3x better' before, but it feels overly simplified. I think how much better you need to be depends on:
- How painful the status quo is for the market; do existing solutions have a perception of being 'good enough' or are folks screaming for something better? (People feel Google is good enough: a -ve for MS.)
- What are the switching costs? (Switching between search engines is relatively easy: a +ve for MS.)