I like to think of myself as the last guy to say a thing can't be done. But it comes down to how it's going to look.
Making a boot that doesn't sag is taken care of way at the beginning to the design and construction process, so anything done now is going to be half a**.
The shoe repair trade has "heel liners" for boots that might stiffen it some, but unless you do something to the front, it's still going to sag. And in either case, it's going to be problomatic, because it needs to be stitched blind, even if done on a long arm patcher. And if your stitch line so much as 1/4" away from the edge, the liner can turn at the edge from going in and out of it. That can be irritateing.
If you wanted to stitch a firmer leather to the ouside, verticaly in the middle of the boots where the sagging is, it might help. But again, it's gonna look bad no matter what.
My suggestion is to have a pair handmade with this as a paricular request.
Or just put me down as someone who says it can't be done...