I agree Robert Johnson is over-rated, but not by much. He happened to tick all the boxes required (by white blues fans) of a "legend" - died young, made few recordings, all seemingly uniquely accomplished, got linked with the idiotic story of the devil and the crossroads.
Those can be the only reason he became better known than his mentor Son House, who had a similar intense personal style - which became the template for the archetypal Delta blues singer: the tortured individual at war with himself and his demons. (Rather than the all-round entertainers, also performing gospel and dance tunes, that most of them actually were.)
IOW, he's achieved iconic status because someone had to, and he had all the qualifications - non-musical as well as musical.
His guitar-playing was certainly not satanically gifted, but it was pretty damn good. I'm still waiting for the OP (though he's probably gone now) to name some of those he considered better. Blind Lemon Jefferson? Blind Blake maybe? Who else? (I'd genuinely like to know if there's some great players of that era that I've somehow missed. Especially if they can wipe the floor with Robert Johnson. Which I doubt...)
I shall tease the OP by saying I think Willie Nelson is over-rated as a legend.;-) (Great singer, writer and performer, but legend?)