@glh
While the physical production plant may still be present, the capability to certify and produce a civilian aircraft may have atrophied. The skillsets between civil and military only overlap to a degree, not completely.
And certification standards would be a huge hurdle - the scope of change being described would almost certainly run afoul of the Changed Product Rule, which would largely eliminate any grandfathered certification credit for much of the design. If you aren't going to get any benefit from keeping an old design, you're probably as well starting with a clean sheet. And I don't think anybody other than the big two is interested in a clean sheet widebody these days...