In speed freestyle, ending your turn in time to get a card is an art.
You would be taking this aspect that great speed freestylers have learned entirely out of the game.
This would change speed freestyle for the worse because there would be one less aspect of skill involved. Everyone would attack until the end of the round, everyone would get a card... it's just going to decrease the amount of skill it takes to win a speed freestyle game.
If we hadn't been exposed to speed freestyle the way it is, we wouldn't be complaining. Your suggestion makes sense. However, the fact is that we were exposed to the current gameplay of speed freestyle, and this would not only change it (which is bad enough), but make it less involved and less skill-oriented.
Remember... it's an art. Don't do this.