I'm unsure about your maths. I was thinking about a different approach (multiply power x accuracy).
Stone edge has 100 base power and 80 accuracy (100x 0.8)= 80
Rock slide has 75 base power and 90 accuracy (75x 0.9)= 67.5
Now I'm not a math expert, but that's 12.5 more base power on average coming from stone edge. Now we can go into comparing the secondary effects of those moves:
30% flinch on Rock Slide
12.5% crit on Stone Edge
So if you take into account the crit rate from stone edge which deals +50% damage and ignores the stat changes (that can be really big), Stone Edge deals way more damage on average compared to situational rock slide.
You are overestimating the power of flinches and already assuming that you are faster than your opponent; if you fail to KO him with rock slide and flinch him, the retaliation can kill you, when stone edge might've brought you the needed KO.
Unfortunately, competitive gameplay isn't all about maths. There's also the argument that Rock Slide with it's inferior base power (25 on neutral and 37.5 on super effective hit) misses the chance to OHKO on many different occasions. Rock Slide is highly situational; it can be used on mons like Aerodactyl or DD Tyranitar (lolol) if you wanna go ghaey and fish for flinches and it's far superior in Doubles due to it's aoe (and flinches).
Keep also in mind, we have gen7 crit mechanics, meaning that Rock Slide doesn't have 6.75, but ~4.1 crit chance.
There's a reason why Stone Edge is preferred by most experienced competitive players and why it's the most common choice by them (aside from mons that got sheer force, obviously). Overall, stone edge is just way better.