Python's a very good example of the problems you get into with a single implementation defining the platform (by fiat, or de facto), even if it's controlled by

Python's a very good example of the problems you get into with a single implementation defining the platform (by fiat, or de facto), even if it's controlled by a completely benign and incorruptible entity. The problem is that then bugs in your implementation become part of the de facto standard that defines your platform. Downstream code is very likely to depend on those bugs. You can't fix those bugs without breaking things. Your platform accumulates warts at a higher rate than it otherwise would. Any attempt to make major changes to the platform implementation becomes incredibly difficult because of the need to maintain bug-for-bug compatibility. All these things have clearly happened to Python. It's one reason why despite massive efforts over a long time, there is no fully compatible Python JIT today. If you follow Chromium development you will quite often see developers consider whether it's going to be safe to fix some bug in Chromium's behavior. Quite often part of the justification that it is safe is "Firefox doesn't have this bug, so major Web sites probably don't depend on it".