It's a decent idea. It IS frustrating (And I mean FRUSTRATING) when you have a teammate dc, but what is more frustrating is when you have someone leave completely because they feed. Still, there is only one flaw with this plan.
If you have a teammate that does bad in the first 5 minutes, they could just as easily unplug their internet cord, which would reset the match. You could also have trolls just (right before the 5 minute mark) DC on purpose.
Don't get me wrong, it sucks fighting a 4v5, but there really isn't much of a choice to consider when dealing with topics like this.