Not to mention the possibility of resheathing. 2-3 layers in most areas usually means old roof boards and usually some leaks somewhere. Usually the hack that put the 2nd or 3rd layer on missed a few things and there are some leaks, creating at least a few spots to be replaced.
To the OP, did any one of those contractors even mention looking in the attic to see the roof boards?
No one has even mentioned asking to go into the attic. I know we do not have plywood as a decking. I have 1x6's. The last guy sent me an email saying they will charge $30 per sheet of 4'x8' of OSB or $3 per Linear foot of 1x6.