* There is a number of implementations of the same platform.
* Those implementations are competing and the market encourages them to be in-compatible with each other.
You can avoid that bad state by having an explicit standard and applying pressure on all implementations to meet that standard. It usually comes at the expense of slower innovation.
But another viable way to avoid that bad state is to simply not have competing implementations. A single canonical implementation also solves the problem of ensuring all users get a compatible experience. It can come at the expense of evolving in a way that doesn't meet user needs because there isn't a competitive incentive to win users.
I don't think there are perfect solutions, but I also don't think it's the end of the world if this becomes a monoculture. There are tons of "platforms" that are effectively mono-cultures and seem to be OK. Every rechargeable tool company has its own battery pack form factor. Up until recently, each laptop company had a different port for the AC adapter.
What I think this is really showing is that the market size of the web is shrinking relative to the size of the browser standards. Mobile apps have eaten up so much user share and HTML+CSS+JS+etc. has gotten so big and complex that the desktop web market can't effectively support multiple independent browser implementations any more. It's just too much work for too little return.
There's maybe an interesting lesson here in not letting your platform get too complex. New features are always nice, but they have a cost. If you pile on too many of them, you may undermine your platform's ability to support multiple independent implementations.