Doing this with mental math for MOA is possible but more complicated, requiring an additional operation to get to the correct answer. If the wind switches direction and magnitude, it will be more difficult to scale your initial hold correctly than to simply recalculate it using the wind number method. Same with elevation, 0.1mil per 10yds change in distance.
There's no bold assumptions, quickdrop can get you really really close, to the point you will not notice the error. Same goes for the wind number method, you will never be able to detect the error due to the method amidst all the other sources of error, for anything less than extreme ranges.
It's actually kind of funny and characteristic of MOA shooters to assume there must be some large error with any mental approximation of the trajectory, because it is so hard to do correctly with MOA

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Seriously though, once you go try and use it for shooting practice, it works so well and you will get hits without ever needing to consult a computer of any kind (other than a rangefinder). This is a little woo-woo but I feel like it also increased my intuitive "connection" to my gun, because I was able to build an accurate mental model of the bullet trajectory. If one wanted to really practice using the reticle to estimate range, they could feasibly shoot pretty damn accurately with no aids at all, just the rifle and some mental math. I guess that appeals to be at some base level haha.