How many flashes in pro matches actually connect and full blind an enemy though?
One of the most dominant teams on a specific map (FNC on Lotus) has been playing no flashes all year. They also played no flash on Icebox, which was another very dominant map for them. It has nothing to do with coinflipping, they just have other ways of achieving the same goal.
Take for example Ascent A site. You are an attacker trying to take the site. Let's assume for a second there are no abilities at all. What you're gonna do is clear every angle carefully. If your opponent is just standing still holding any of these angles, you probably win. So a good player won't do that. If they play say, behind dice, they will jiggle occasionally trying to catch a timing. There's a very good chance you clear that angle, don't see them, and then just as you're clearing the next angle, they peak out and kill you.
So now let's add abilities into the mix. If your opponent plays like this, a flash is gonna have zero impact. It either won't catch them, or it catches them on a jiggle and they simply tuck behind dice for two seconds (which they would have done anway), and you won't get any info (unless it's a Skye flash). A prowler could be much more effective in clearing that out, because they can't out-jiggle it. Any AOE ability behind dice will also likely lead to a kill because they have to swing and you'll be ready for it.
That doesn't mean flashes are useless of course, I watched Acend play Bind earlier and they had some crazy Yoru flash tech, especially when you pair the flash with something like a Fade eye to force them to look at it and pop the flash right in front, guaranteeing one of the two abilities to hit them. But it's not like flashes are the only possible ability, or even the best ability, for denying angles.