Those that think the Mk1 eyeball is enough have probably missed hundreds of potential conflicts in the past but just don't know it.
Who has advocated that?
The Mk I eyeball is the basic method of detecting and avoiding traffic in VMC/VFR conditions beginning with the dawn of aviation and remains so today.
Everything else like TCAS and other aids....assist that Mk I eyeball.
As Hot and Hi notes....those secondary methods may prevent the MK I method being successful but none the less he was looking outside trying to see the traffic.
If none of the additional methods, equipment, or services exist....what do you do to achieve traffic separation in the end.....look outside of course or gamble blindly relying upon the small aircraft big sky method?