[msclrhd]

If there's a bug between the specifications and Chrome, what incentive is there for Chrome to change? How do you check/verify the specifications if there is only one implementation?

Given a single implementation, what incentive is there to develop open standards?

How do you innovate (in standards, JavaScript performance, rendering, etc.) if there is only one group working on them?

Case in point: Firefox is the only major browser to provide MathML support. Where is the incentive to get that implemented in the other browsers, especially given MathJax?

Where is the competition to innovate in things like web tools (layout analysis -- esp. for flexbox, etc., accessibility property (WCAG, WAI-ARIA) navigation/investigation) given a single implementation?

[cdata]

This doesn't address the full scope of your point, but it is worth mentioning that Chromium/Blink engineers contribute to and rely on https://web-platform-tests.org/ in order to ensure some level of conformance and interoperability.