He was already a top CB when he was at Southampton which is what got him a big money move to Liverpool in the first place. That was the 15/16 season, or 16/17 at a push if you want to argue he wasn't amongst the best CBs in the league in his first year at Soton. Performed immediately after arriving at Liverpool and has been in "best CB in A, B C" or whatever discussions since our high-performing 2018-2020 period. Recovered from an ACL injury at age 30 or so, which are notorious for losing speed and mobility especially at a later stage during a career, then is now back to being one of (if not the) best CB in the world after a "down year" which came after a season where his team competed until the last week of the season for a quadruple, and the entire squad downed tools after a draining season.
If that's not a sign of consistency I don't know what is. People are putting Jaap Stam, who had 3 years in the PL, in the same tier as Van Dijk, who has been at the very top for 8 years in the PL by now.
And that doesn't even get into the argument that personal accolades and trophies as well as team trophies elevate a player's status if you talk about "legendary" or "the game's greats". Van Dijk has basically won it all, as opposed to some other big names, and has been the epitome of a big-game player recently, I think he's been MOTM in our last two domestic cup finals and our CL final win. You can take that out of the equation if you purely tackle the discussion from a player's quality perspective, but those things also count for something (not everything, but something).
The consistency argument is really crazy to me, as if Vidic, Ferdinand, Terry never had "down" seasons based on their incredibly high own standards. Those just get looked past once a career is done because people only tend to remember the good stuff imo. Kompany also had very bad spells during his time at City even though people will only remember the titles, the Leicester goal, the header against Utd, ...
Shifting to attackers, a player like Hazard is considered one of the best wingers the PL has ever seen and he had a down year every other season.