The customizer really is quite polarizing in the WordPress world. People often either love it or hate it. We're definitely getting some more of it each release.
Nick Halsey has submitted a proposal to put WordPress menus into the customizer (here's the actual feature plugin on w.org). People have opined on this proposal, and many of the opinions are not so positive.
Squished, difficult, and pointless are some of the words I've heard folks use to describe it. They have some merit, for sure.
It's good to remember why this is being proposed in the first place. And the proposal (in different words) basically sums it up like this: it's faster (thanks to no page reloads), it's simplified (which is obviously subjective), and it offers a live preview.
Seeing the result of what you are doing, while you are doing it, is always good in my opinion. However, I understand the push back on this.
From a client services standpoint, Andrew Norcross makes some pretty compelling arguments:
The decision to deprecate / remove the existing menu is a horrible one IMO. While I don’t like the idea of putting menus in the customizer to begin with, removing the existing one poses a lot of problems for existing education / training done with users. I have trained at least 100 clients on how to build and manage their menus, both in person and with online / printed manuals. Removing this creates a new set of required training for both the menu area and the customizer as a whole, not to mention opening up the idea of “can I now customize X part” on sites that were built to specific scope and needs.
I hope the core team reconsiders this.
Sheri Bigelow notes in response that she's done user testing and current menus aren't such a great UX either. I don't need user testing to know that, but do we know that the new setup is a better UX?
Chris Lema brings up a worthy point, that the crampedness of the new method could be troublesome for some users. I would hope the Accessibility team will get a solid crack at this.
And as Chris and others have noted, this could also be weird if now the link to modify menus gets buried another level under the “customize” option in the admin. Surely that could be prevented quite simply by keeping existing menu options.
I think there are good points both for and against the new feature. Perhaps the push back is a sign that it needs to bake longer. But I don't think we should be afraid to experiment with WordPress because we already taught users the current way. Training can be a pain in the butt, yes, but fear of change brings stagnation and stagnation makes software die.
We'll find out soon if this gets approved for core. Also remember, even if this makes it to core, it can and will be iterated on to improve it, and you can be a part of that.