For the old IE, Firefox happened (and other as well).
My purpose isn't (just) pedantic. You claimed "No, forcing everyone to use the same engine does not give the incentive to refactor it into something nice. It gives the incentive to create a black market.", but then your example shows an incentive to make a superior product. It starts making a lot less sense in Blink's case if you change it to the more accurate "No, forcing everyone to use the same engine does not give the incentive to refactor it into something nice. It gives the incentive to create a superior product." Why not a superior product that starts with a Blink fork?
IE did not force anyone doing anything, even if it had a de facto monopoly, therefore Firefox was created, because people chose to.
In this scenario I'm discussing it would not be allowed to create a Firefox, because everyone was forced to use Chrome, hence a black market is created.
Also the fact that this is even an issue is a product of HTML5, etc. leaving a lot of room for undefined behavior and being easy to misinterpret/misimplement. You don't even have to purposely bend the standard to try and create a monoculture; hard to predict implementation differences happen organically.