The problem is they need to have some sort of tie-breaker. Over 3 games Groups are often level on points. The next tie-breaker is goal difference, which is also tied. The next tie-breaker is goals scored , which is also tied. Making them play until there's a winner only makes sense because they'll end up playing each other. However, this situation can happen after the 3rd game when the 2 tied teams weren't playing each other in the 3rd game and they could be 1000 miles apart depending on where in Russia they're based. Using cards as a tie-breaker is no worse than going by a coin flip or 'Official FIFA rankings'.
I guess it really don’t matter since both are advancing anyways. I’d have a far bigger issue if it was deciding one advancing and one going home.
