I call old wives tales. I'm a close-to-20-years Apple user. The walled garden has never been a problem, more of a strength (integration, cohesion, etc).
Especially today where everything is a subscription, there has never been LESS of a walled garden in that regard. I can move anytime (and I do use also Windows and Linux) and I wont have any issue with OS X/iOS.
I just wont have my apps -- which has been the case for every OS ever.
My music is in Spotify and Apple Music, my video is Netflix and Amazon Prime and Apple+, my email is Gmail, my eBooks in Kindle for Mac and iOS, I talk with friends with Skype, Facetime, Messenger and Whatsup. Even my iCloud apps (Calendar, email, etc) are available from any browser.
Some "walled garden".
The reason I don't want to move from OS X/iOS is cohesion and usability. Despite BS like the 3+ years MBP keyboard fiasco or the not-really-useful BS touch strip, it's still the best HW/SW combo for my kind of computing (programming, lots of UNIXy stuff, lots of video and music as well).