Even if it could rear up and balance like that, the energy expenditure vs calorific gain seems like a losing proposition. You're talking about raising the center of gravity of it's 40-ton body mass by 10-20 feet just to grab a very small mouthful of low calorific leaves.
I'd guess the reason the sauropods had an extra long neck was rather so they could AVOID moving as much as possible - stand in one place and just swivel neck around to graze a large area.
So it looks like this pose is based on anatomy, not biomechanics, and the one rigorous biomechanical sauropod-rearing study that exists didn’t even test this genus - which means the rearing question Mamenchisaurus is unresolved.
https://en.wikipedia.org/wiki/Mamenchisaurus
https://reptilis.net/DML/2009Apr/msg00036.html
https://onlinelibrary.wiley.com/doi/10.1111/pala.70019
ALSO, consider how stiff their neck was, it could very well have spent most of the time grazing on the ground, like you said!
Sidenote: you underestimate the cardiovascular cost of pumping blood up a 5-15 meter neck. It’s not at all clear that a rearing strategy is more expensive energy wise. In their case it’s less spending energy to standup than just leaning back to let their skeletal structure and center of mass do the work.
Mostly I think this pose is a matter of logistics. They probably just had more vertical space than horizontal to work with for this exhibit. Even though they’re fiberglass, the casts for these guys run well into the tons per skeleton so it can be challenging to mount the armatures in an existing structure and it turns into a game of fossil tetris balanced by the cost of structural support modifications needed (there almost always are for a fossil of this size).
It'd be interesting to see an accurate energy analysis of the calories needed to do this. Even if the animal can position itself into a teeter-totter position with center of mass over the pivot/legs, it would still be using muscular energy to straighten up and extend, and then coming back down can hardly have been lossless - it'd be a combination of again using muscles to come down in a controlled manner (and not destroy it's front joints!), and then a final plop down which would transfer kinetic energy into compressing the landing spot... all for a mouthful of leaves.
[1] nature video starts with example: https://www.youtube.com/watch?v=3XzQ4BQe4fM short clip: https://www.reddit.com/r/NatureIsFuckingLit/comments/19bge4y... longer clip with two: https://www.youtube.com/watch?v=Jpxgqu_Cfkg
At least an elephant, having a trunk, can pull down a whole branch and make the effort worthwhile as that first video shows. It seems that a sauropod with its tiny mouth for grabbing wouldn't be able to do that, so the outcome would be more like in that last video where the elephant was only able to grab a couple of leaves, which can't have been a calorific win!
> Two years ago, Citadel's Ken Griffin paid almost $45 million for a stegosaurus skeleton, making it the most expensive fossil ever sold at auction. So why are dinosaur bones joining the collections of millionaires instead of museums? How does the private market for fossils actually work? And how similar is it to the market for art and other antiquities? In this episode, we speak with Salomon Aaron, a director at London-based gallery David Aaron, where he is the gallery's in-house broker for dinosaur fossils. We talk about how fossils are found and priced, what it's like to work alongside dinosaur hunters, how his gallery identifies potential buyers, and why Joe thinks something about the birds-to-dinosaurs evolutionary pipeline is off.
* https://www.youtube.com/watch?v=hf4nv3ggdqE
* https://podcasts.apple.com/ca/podcast/inside-the-booming-mar...
* https://omny.fm/shows/odd-lots/why-dinosaur-fossils-are-sell...
[1] https://obscuredinosaurfacts.com/blog/post/2019/08/31/bronto...