Without a second browser engine to test pages in, it's really hard to know what is a bug and what is intentional. Devs don't know all the specs by heart. They write whatever happens to work for them, but sometimes they accidentally depend on obscure edge cases in the implementation that were never meant to exist.
In the long term it's paralysing for the engine maintainers, because any change in implementation could be changing some subtle behavior that breaks some pages. W3C requires two independent implementations, so that they'll share intentional behaviors, but hopefully their bugs will differ.
The single-engine Web will be as fun to maintain as Windows: Windows 11 Explorer has a shiny new context menu with an option to reopen it as an older, uglier context menu, because Microsoft couldn't touch a line of code of the old context menu without breaking apps.
I know this isn’t everyone’s experience, but I believe adding IE11 support to my personal website improved the quality of my code.
There were a lot of things I hadn’t specified explicitly, and for whatever reason Chrome/Webkit and Firefox just happened to make the right guesses. IE11 did not, so I had to go back and be clear about exactly what I actually wanted.