No one is arguing that Smash 4 isn't competitive, from what I've seen.
The edge mechanics are way more involved than you described. In Smash 4 your options off the edge are indeed limited, but most characters have insanely good recoveries and can get back to the edge for free. Sure, at that point they can't threathen ledgedash, but even if they get punished for a roll on stage they probably won't die until 150%.
As for Melee, very few players are able to abuse the ledge effectively. Even then, the edge is far from an advantageous position. Camping the edge may be safe, but getting back on stage is not: most characters don't have good ledgedashes, and you make it sound like there's no risk involved, when the penalty for mistiming it is death. It's not like ledgedashes are impossible to punish, either.
Other ledge options suck in Melee compared to Smash 4: roll is slower, and get-up attack or normal stand-up have additional lag over 100%.
I mean, I kinda get your point, but it seems a little strange to conclude that Smash 4 is the most competitive because of the edge mechanics, when in reality Smash 4 and the previous games are too different to compare in this aspect.