I can’t even begin to imagine what a match with all Montanas or Galileas would be like. I mean, it would be such a massacre it would be unplayable. Or a map with all Marquis’. In fact, a map with one team being any one character would be really bad.
Forcing yourself to pick one character is a pretty good strategy. Someone used Oscar Mike, so let me try Reyna. Someone picked ISIC. No problem, I guess I could try Miko and see if I like him. Picking one character forces the person to think outside the box. Yeah, sure, it may suck that someone took a character you are used to play with, but that gives you the initiative to try and master another character so it doesn’t become a problem. It’s why people have “alternatives”.
If Overwatch wants to go ahead and have multiple characters in one match, that’s perfectly fine. It’s just one of many things I’m sure that differentiates itself from Battleborn.