The thing is, an open ecosystem that allows users to run any software they want on their devices already exists, including on Apple devices, in the form of the Web.
And despite all of its flaws it's the most successful software deployment platform the world has ever seen, to the point where even Apple can't afford to not include it on their platforms when we all know they'd rather not, because it's simply too valuable to consumers and would make their devices obsolete if they didn't include it.
There's no central gatekeeper for the web, and sure, it has its dark corners, but it has not devolved into absolute chaos as you so adamantly suggest all such ecosystems would, and continues to deliver such an incredible amount value to consumers and businesses alike.
The web was only possible because it was developed in an era before everybody was using closed down devices where the manufacturer dictates what software you can run on it. And at this rate, if the web ever falls out of favor (and Apple for one is doing everything it can to make sure it does), you can definitely be sure that nothing like it will ever be allowed to exist again.
As a community of founders and makers, I'm sure we can all imagine what a sad world it would be to live in if we had to first convince some platform gatekeeper that our idea is worthwhile and make sure our ideas don't conflict with their interests before being allowed to turn our ideas into reality and deliver them to users, let alone having to pay a cut of our revenues to them for the privilege.