For a lack of better words, I think too many people are getting their metaphorical undergarments in a bunch when complaining about the hamburger menu. I'm not happy about the UI changes going on in the Technical Preview either, but I'm willing to see what Microsoft is planning to do with the final product and give as much information, opinion, and feedback as I can to make sure the final product is a usable system in my eyes.
However, I think people are forgetting that UX guidelines are just that; guidelines. From a consistency standpoint, it makes sense to me to go about this route in the future with the convergence of Windows-as-a-Service for all sorts of devices. If apps in the future have the power to be run on a phone, laptop, desktop, tablet, convertible, all-in-one, Xbox, and other platforms (as shown in iOS and Android); it makes perfect sense to me to unify consistency between all the platforms to make sure a Microsoft-designed app is a consistent look and feel regardless of what platform it may be running on. It's in Microsoft's interest to make their apps look consistent across platforms to minimize the confusion users come upon when using their software from device to device, and to showcase their brand look across audiences. It should feel like a Microsoft product whether you use it on Windows 10, iOS, Android, and any other operating system Microsoft may want to pursue.
To encourage a consistency across the OS, I think it's encouraged that app developers follow that guideline, too, but I think it's an encouragement and not a requirement. <Hub> controls in XAML/C# are not going to be suddenly gone and stop working, and <PivotControls> for older apps that haven't transitioned to using <Hub> and other content controls aren't going to be going away anytime soon. People will still be able to develop applications for Windows in the ways they want, whether they want to jump in to the adaptive UI changes in code coming, or if they still want to use common shared code and different projects for each platform to define how the UI will look dependent on device.
I'm not saying it's all roses and honey; this flexibility added to the system can create user confusion as they traverse from app-to-app if and only if the design language between apps isn't unified, or if and only if developers won't accept following a new UX. Redesigning an app is expensive in time and resources; but thankfully it's not as expensive as it could've been, especially if it's already a universal app, and especially if it nicely follows MVVM; and thankfully the new changes aren't going to break existing universal apps, to my knowledge.
The best bet for now is to keep voicing opinions and keep giving feedback. However, I don't think complaining about how things are changing is helpful at all. It doesn't mean that the complaints are valid; I just don't think they're helpful, and that it seems very, very myopic.