You ever fight a good, or even a decent rosalina? Ive had a best of 5 set where i won 3-0 take almost 20 minutes (ya know, time between matches) because almost every dame game goes til the last 30 seconds in a 2 stock 6 minute. An extra stock and only 2 minutes would give rosalina the ability to time out people, and im not saying than lets make it a 9-10 minute timer, because nobody is going to sit through a 1 hour grand finals bracket reset.
On the flip side of things, smash 4 has a rubber band effect called rage, so regardless of stock start count, its almost damn near impossible to win if you lose the first stock. Lets say you quickly take back the stock and tie it up. You are still faced with the same situation for the last stock. 9 times out of 10 though, it gets out of control because rage eliminates the neutral game and also makes it harder and takes more damage for the person who lost the first stock to even clean up the other guys first stock.
To sum up, this game is really all about the person who takes the first stock and maintains stage control and pressure for the second stock for the victory.