Don't confuse ecosystems with walled gardens. Not all ecosystems are walled gardens at all.
Correct, though in the case of MS/Windows/Xbox it is a walled garden. Off the top of my head Amazon seems to offer the ecosystem with the fewest walls.
I've read this entire topic and I'm surprised to hear little mention of Education and the creative professional arena. Even Steve Jobs back in 1997 realised these were Apple's 2 strongest markets. And I believe today they still are.
Apple lost a lot of the EDU market around the turn of the century because they couldn't compete on price. I think in the last few years they have rebounded though. The creative market is slipping away from Apple as well. The long, long time between Mac Pro models and horrible roll out of FCPX has more people than I've seen in the last decade switching (or at least specking out) non-Apple workflows. Places might not be dropping Apple entirely but many are/will no longer be Apple-centric.
Apples biggest sellers for the last 6 or 7 years have been iPods, iPhones and iPads.