I mean, yes, redoing every pose with a mannequin is awkward and time consuming, and some poses you will never be able to accurately copy with it.
The mannequin is a nice toy, that can very effectively introduce a beginner to a certain type of simplification of the form. I feel it has an in-built expiry date. You buy it, you use it a few dozen times or so, you get the message, you gradually stop using it. In the end it mostly stands around on the desk or a shelf, until every now and then you really struggle with a specific problem of foreshortening, and take your chance of maybe triggering your brain better with the mannequin.
Given that a mannequin isn't crazy expensive, I think it's fair to have one, even if you won't be using it forever or very often.
What I don't see is how a 2-D set of shapes would do a better job than a mannequin. They will just be even more constrictive in their use cases, and they won't even look decorative on the shelf.