Why create unique strategy with X heroes, if you won't even be able to use them? You are not meant to be stubborn and play with that X hero whole game, if it doesn't work.
You are just using very weak comparison and you probably know it, but think about this: MOBAs (Dota2/LoL) have over 100 heroes, where as Overwatch currently stands at 30. Lets do Double/Triple ban for OW too then, if you say 30 heroes is enough so why not just reduce them to 26 or 24? Ban 6 tanks, no problem just play that 1 remaining tank.
Also you clearly didn't read word "SUBSTITUTE". Why have subs, if you don't even know, which heroes would be banned? And since that system would have to be in ranked too so good luck, if you really want to think it would currently work for OW.