As for multi-level menus, most applications avoid any problem by including only one, or, at the most, two sub-levels (ironically, Unity requires far more mouse-clicks down the levels than any other modern desktop environment). I also seriously doubt that many people's reading ability is seriously strained or distracted by menus, given that most menu items consist of one to four words.
Similarly, arbitrary organization is usually confined to less commonly used items, such as configuration. These days, you will hardly find any application that doesn't start with the File and Edit menus on the left, and end with the Windows and Help menus on the right, no matter what operating system you are using.
Where convention is less fixed, users quickly learn each development team's preferences -- Mozilla, for example, places Prefences in the Edit menu, while KDE-based applications favor a Settings menu. All of which means that next to nothing of Shuttleworth's rationalizations stand up to casual scrutiny.
Even more importantly, Shuttleworth himself admits that HUD only replaces the easy accessibility of menus. Providing a complete map of features, which is the other advantage of menus that he cites, is by his own admission not addressed by HUD at all. Yet HUD's implementation is going ahead, even though it is only half a solution.
Shuttleworth notes that another alternative to menus is Microsoft's so-called Ribbons, a kind of combination of menus and taskbars. However, the advantage of HUD, he says, is that it avoids such clutter -- an obsession that seems to underlie much of Unity's design, even though few users complain about the clutter of menus or toolbars.
Nor does Shuttleworth look closely at earlier efforts that included some aspects of HUD. If he did, he might note that applications like GNOME Do or KRunner, in which users type in commands with auto-completion, appeal only to advanced users who know what they want. Less experienced users are likely to avoid such tools.
Similarly, Windows has long had menus that alter according to your patterns of use. Most people turn them off in favor of fixed menus where they can find items more easily.
No doubt menus will be replaced some day. I doubt, though, that for the average user, their replacement will be HUD or even voice recognition, which is considerably better than nothing for those with accessibility issues, but still far less efficient than menus. Although menus are hardly elegant, they have a kind of lowest common denominator efficiency that any general replacement needs to equal -- and HUD doesn't come anywhere close.
Unlike Shuttleworth, Cinnamon's developers make no reference to any user testing that informs their development directions. However, Lefebvre does offer a rationale that most users can support.
He writes that Linux Mint is working toward an interface that ensures that the "computer works for you and makes to easy to be productive." As a second principle, he adds that "things aren't hidden away but easy to access." He also emphasizes the importance of configuration "to make you feel at home."