I wouldn't say it was dictatorial - Metro was a strong set of guidelines and templates to help developers get a good enough looking app without the need of a UI designer (can't say it worked though).
Nothing was stopping you from actually doing a hamburger menu yourself - other apps have done it - see Facebook and Barclays.
To me the decision to move away from Metro feels like it wasn't made by the design team. I think it was mostly driven by engineering and the business and of course time constraints to launch Windows 10. I can understand it from that perspective, but I don't necessarily think it will help adoption for WP.
Will developers now suddenly consider to port their apps, just because the UI is similar to iOS / Android, implying that it will be easier?
I doubt it.