You wouldn't if you look more deeply at this. He doesn't push for simplicity but for horrible complexity with an enormous stack of polyfills, ignoring language features that would greatly reduce all that bloat. .
That's also a problem. I've written JS that would work on any browser from the latest ones all the way back to IE5, and I'm not even a professional JS developer. It's not hard.
Maybe "professional" is the problem: they're incentivised to make work for themselves so they deliberately add this fragility and complexity, and ignore the fact that there's no need to change.