Toni, as stated D750 was cutting edge 3 years ago and it still is a pretty quick camera today. Your 5D Classic was sluggish 12 years ago, when it was released!
D750 is a better all arounder FF body than anything Canon ever made in digital.
Edit: 6dm2 is a serious competitor though. Disregarding that it's also more expensive and came out 3 years later. Still is not a clear winner over D750, mind you. Why 6dm2 and not 5dm3? Sorry, can't reply while I'm using the tilt screen to shoot from anything but eye level.
Edit: While ergonomically 6dm2 would compete with a D750, image quality from the Canon sensor would still be behind the 3 year old Nikon with the 5 year old Sony sensor.
D750 is a better all arounder FF body than anything Canon ever made in digital.
Edit: 6dm2 is a serious competitor though. Disregarding that it's also more expensive and came out 3 years later. Still is not a clear winner over D750, mind you. Why 6dm2 and not 5dm3? Sorry, can't reply while I'm using the tilt screen to shoot from anything but eye level.
Edit: While ergonomically 6dm2 would compete with a D750, image quality from the Canon sensor would still be behind the 3 year old Nikon with the 5 year old Sony sensor.