When has anyone ever claimed that some single stat should capture something as complex as "who was the most important player in the NBA over the last five years"?
That's an idiot strawman that no one has ever run with. No single thing can capture that.
But I'll do it anyway, just for you:
Lebron has led all players in playoff plus/minus (+543) from 2014-2017.
Lebron's presence makes a bigger difference in whether his team will outscore the other team than anyone else in the NBA.
There are ways in which on/off can be skewed (such as if you only play for the 1st unit, or if you are always on the court with another particularly effective player, or if you backup is particularly bad). But since Lebron plays more 2nd-unit minutes than any other starter, those don't apply. So the benefit is legit.
Or here is a really simple one for the rock-brains:
Lebron has led all players in playoff scoring from 2014-2017 (2,292 points) while shooting above his team average (60% eFG).
That's a fairly simple statistic, but you probably can't find any four-year period in the last forty years where a player has led the NBA in playoff points with an above-average FG% and not had an argument for the best player in the NBA over that time.
Lebron has led the NBA in playoff assists over that time (565) while maintaining an above average assist-to-turnover ratio (2-to-1).
Then you could get into analytics, like showing how much better his teammates shoot off of passes from him, how much more poorly opposing players scored in various situations against him, etc.
But that would first require you to know what analytics even is.
First off, by the end of the regular season Lebron WAS the 3rd-6th best player in the NBA last year. The actual players who were playing the best in the regular season last year were Durant, Kawhi, Harden, Lebron, Curry, and Westbrook, and Lebron was somewhere around 3rd-4th in that group.
Second of all, PER AND WIN SHARES AND BOX SCORE PLUS/MINUS AND WINS ABOVE REPLACEMENT ARE NOT ANALYTICS.
Stop being such a rock brain and using words that you don't know what they mean.