Slater was no doubt an exceptional talent who revolutionised the fullback position and was elite in many facets of the game BUT a lot of his play revolved around supporting and working off opportunities / execution by others... Would Slater have been as electric if he didn't play his entire career with playmakers like Smith, Cronk and Thurston creating space, setting up plays and facilitating opportunities? In my opinion, probably not.
Lockyer was the main man. He was the playmaker and he created opportunities out of nothing in key moments from both fullback and from 5/8. He was also a passionate captain and leader and figurehead for every team he played - from Broncos to Australia. He took poor teams and single handedly elevated everyone around him. Slater was excellent absolutely, but Lockyer is a through and through legend at 2 positions and as a captain.
I understand the sentiment that Lockyer can't be the 6 due to Wally (I'm too young to ever have watched him play), but there is absolutely no justification or realistic way that Slater could be picked over Lockyer at fullback.