I ran this as a controlled test in my house using 500mm F4 and found exactly that...the 5D4 with 1.4x will outresolve the 7D2 bare, and the extra high ISO performance will make up for the lost stop of light, plus a wee bit more. The 7D2 with 1.4x will outresolve them both, though.
So, I kept the 7D2 (I shoot mostly birds) and still use it when I need to crop a lot and when the light is good. When close (like feeder setup or large, tame birds) or low light, 5DIV.
What he said