This isn't really up for debate, it used to work 'ok' with its non-predictable 'enlarge' functionality, but now its forcing us to work in a tablet environment with what should be a desktop.
And sorry, NO, it is just one option; one that needs fixing.
Oh, boy, here we go. That moment when someone says "no, it's not open for debate, it's objectively as I say it is". Um, no it's not. And this is the perfect example - for me, it didn't use to work 'ok' - as you yourself said it was non-predictable and I never used it, NOW it works the way I want it to. I love the full screen mode, even on my 27" iMac. It's not about space, it's a bout the app being distraction-free. And now they added split view which makes it even better - in fact, I have El Cap on both my iMac and my MacBook and I use the split view to drag things from Safari to Evernote on my iMac more than on MacBook's smaller screen.
As for why they didn't add the option to change it, others have already explained it - if they added everyone's desired option, it would make a total mess out of the System Preferences app. But, ok, I guess they could've added that one little option to change the green button. They didn't - probably because 1. it already has the old functionality with double-clicking the title bar, 2. most people like how it works and it's not really that important.
I have several of personal issues with OS X, only I don't make a big deal about them. For example, I hate it how the Favourite Bookmarks address bar drop-down in Safari doesn't work in a new tab while we're in the Top Sites screen, so I can't use both. It's driving me nuts and I even sent my feedback to Apple. And they did NOTHING! I mean, come on people, why aren't you guys talking about this instead, it's much more important than this green button nonsense, right? I mean, objectively it is -
it's not open for debate!
Finally, as for that 'Apple is iOS-ifying OS X" stuff. That's just silly. Apple is doing its best to provide the best experience for every device, unlike Microsoft who tried to make it the same for everyone. Apple is adding and improving features that are unique to the Mac - like the new, even better Mission Control. They didn't throw out MC to make it look like the iOS app switcher, they made it better and even more "Mac" like. But, of course, people see just what they want to see - Apple tweaked one little button and there it is: they're dumbing down the OS to make it look like iOS. No they're not, they're making them work well side by side, while retaining what makes them unique.