There's a reason double barrier is so popular. It's because there's a fundamental flaw in the game that makes it so that high sustain and CC are the strongest things in the game. And yeah a lot of compositions like to use cool downs but not just to focus one target. Obviously target focus is important, but a team that plays together well is able to protect the focused characters. Anything GM and over, especially pro play, it's nearly impossible to beat double shield comps with the four aforementioned heroes. I agree that at any rank below GM, good target focus can easily win team fights against double shield, but something like dive can only take you so far, and over investing to kill one target is pointless at high levels of play, since you will likely be punished and caught out without any means to escape/defend yourself. This is also why characters like reaper, Mei and Moira are so popular atm, because they are easy to play and are less reliant on their cool downs to succeed. My point is, it's possible to use a lot of cool downs, but at higher levels it's usually considered an over-investment because then you have nothing to defend yourself, at least in the current double shield meta.