The very start of this thread gave an example of lock-in, being a custom API to get 3D performance to match other vendors. Judging by the size of the gaming industry, 3D performance is something people are interested in.
So Apple's grand plan is to get better gaming performance to lock people in?
First of all, gamers don't prefer Mac in the first place.
Second games are transient. You play them, and then you mostly forget them as new ones come out. You could jump to a new platform (that would be Windows) and buy new games and it wouldn't be a problem that your old Apple stuff doesn't work in Windows. So that's not much of a lock-in.
Third, if with the custom API they get "3D performance to match other vendors" then that's not a tempting proposition that can lock people in: it's "a match".
If, OTOH, they got performance many times that of other vendors with Metal, then that would be the GOOD kind of lock-in, the one where a platform is so much better that you use that.
No, the whole point for me is keeping the weeds out and have a comprehensive experience where stuff works together well.
It's 2015, there's nothing much to keep me locked in in any "walled garden".
I can use another client for my IMAP emails (or export and re-import them in 20 ways), I can move my photos out of Photos app, I use Chrome to begin with but I could very easily export Safari bookmarks too, my iTunes music files are not locked, so I can play them on Linux/Windows, etc.
What the apps? Those were always non portable between platforms, walled garden or not. And in the last 10 years, a lot of those are run on the web in the first place, or offered for free/near free anyway with the OS, or are subscriptions (e.g. the Adobe Suite, Microsoft Office 365, etc.), so I can move those about freely between OS X and say Windows.