Should a person be eligible for SECDEF or other high-ranking civilian posting after serving on the board of major Defence Contractors?

Why not? If the person is a highly successful manager in the private sector, hopefully they will be a highly successful manager in the public sector. This is a political appointment. He or she should recuse themselves from any contracting decisions involving their previous employer. There’s DoD contracting rules about this.

I think so

I don't see an issue as long as they severe those ties when they step into the new job. I could see a conflict of interest if someone still maintains affiliations and is profiting from decisions made as SECDEF eyc.