You do not see it is the issue.I've seen it. Used it. Recommend it!
Yes it will be expensive, and heavy and ridiculously cool.
So far, I don't see a problem.
sigma 200-500/2.8 lens is a monster at around 18kg body mass. A 100-500/4.0 FF lens would be a pushing it at around 8kg -ish Still handholdable for you? Now imaging zooming while handholding at the same time. Not a practical lens.