But just taking an MBT crew as a fixed (for our purposes) example, then why would we expect a de facto lowering of standards with females in tank crews?

Aren't tank crew are trained and evaluated based on performance in maintenance and various combat-relevant tasks? What would the difference be with female applicants included in the mix, unless you make the specific prediction that female tankers will be more likely to 'burn out' in a sustained combat environment (or other such things that might be more difficult to catch in training)?