You are certainly entitled to your opinion. At my company we use Mint Mate on all desktops. It's clean, simple, works great, and doesn't require training for employees who have never used Linux.
How much training does it actually take to learn the GNOME 3 interface though? I use to train people at a call center how to use an old terminal based DOS application to fill out orders for sausage in 3 8-hour days. But gnome 3 is a lot more simple. Surely 8 hours is enough to teach about work spaces and how to press the windows key?
It's funny you almost never hear about companies having to train staff to use a company smartphone. I guess all smart phone interfaces are pretty similar though.
Then the names of the programs (and how to find applications in the activity view). Many people navigate by position in menu.
Then the change in Alt-Tab/Alt-` behaviour (could be fun) and the new workspaces (not many use those)
Then just being around when odd combinations of old workflow and new one happen. Minimising a window and forgetting the application is running. Nothing to click on to get windows back &c
I'm not defending anything here, just answering the original question.
Also, the assertion that GNOME 3 is extendable is funny. It's probably the least so out of the major GNU/Linux DEs, as the developers have been hellbent on pursuing "brand coherency" as of late, and in general dissuading from tampering with defaults.