I really don't get why people pull this "if you don't like it, leave" argument (in everything from politics to tech). There are lots of reasons someone might want to use a mac (I do), one is perfectly entitled to complain about aspects of Apple's approach to try to make the argument for change.
Avoiding a walled garden can only happen before you invest in the garden. Once you have spent money in iTunes or the store (or e.g. even have a long list of contacts in some IM client) you are stuck unless you are happy to lose those purchases/data.
With how much data has moved off of devices, losing data isn't a huge concern anymore. My mail, calendar, photos, etc. were already hosted elsewhere on services with cross platform support.
Cost of replacing apps is the bigger issue, and for me it worked out to something under $50. That was cheaper than the (~$200) cost premium of a 64 GB iPhone over the Z3C and a microSD card.
EDIT: If you've tied yourself into HomeKit or Apple's photo system, things would be worse. Maybe I'm atypical there?
My rationale was kind of along the lines of avoiding the sunk cost fallacy: just because I'd been invested in the Apple ecosystem for 20-some years and buying iTunes stuff for more than half a decade didn't mean that I should just keep doing that when I was unhappy with other things. But that cost-benefit calculus might work out differently for other people. The lock-in is very real if you don't want to lose your stuff. It's a reason that I still refuse to buy movies from any online service. Only discs that I can rip or DRM free downloads for me from now on, so I don't end up walled-in again.
Eventually I was able to rescue all my protected music at least by paying what ammounted to a $25 ransom for a year of iTunes Match (or whatever they call it now) so I could download DRM free versions of the music I'd already paid for.
It's been a good experience except for one thing: Apple delivers no drivers for the integrated video, disables it in EFI from Bootcamp, so it runs hot and has a short battery life when not in OS X. I haven't seen anyone reporting success working around this problem, and it's been going on for years for us "switchers" to Windows.
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.
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.
Argument By Dismissal:
an idea is rejected without saying why.
Dismissals usually have overtones. For example, "If you don't like it, leave the country" implies that your cause is hopeless, or that you are unpatriotic, or that your ideas are foreign, or maybe all three. "If you don't like it, live in a Communist country" adds an emotive element.