This is a good question. Let me give you an example.
Let's imagine for instance that Google develops something called "Google Pay". Let's now imagine that WHATWG works on something called "WebPay".
In a multiple-engines world, there would be pressure for Blink to support (and keep supporting) most major credit cards & payment mechanisms to let users pay using WebPay.
In a single-engine world, one single manager at Google will have sufficient power to decide that WebPay supports only Google Pay.
Feel free replace Google Pay and WebPay by any other technology strategic to Google.
It's still true and always will be. Blink is not a browser any more than the Linux kernel is a standalone operating system by itself with no attached software or applications.
Let's imagine for instance that Google develops something called "Google Pay". Let's now imagine that WHATWG works on something called "WebPay".
In a multiple-engines world, there would be pressure for Blink to support (and keep supporting) most major credit cards & payment mechanisms to let users pay using WebPay.
In a single-engine world, one single manager at Google will have sufficient power to decide that WebPay supports only Google Pay.
Feel free replace Google Pay and WebPay by any other technology strategic to Google.