Amazing topic you got here... I think staying focused for a stretched period of time can sometimes get tricky as there are lots of factors that can take away the focus. However, there are rare people who still have the capability to stay laser focused all the time.

In Sports, especially in football, you are right, as a player doesn't necessarily needs to be focused all through, he only needs to be alert and know what to do when the time is right and when to switch it back on. This also helps save some mental energy, not much, but goes a long way over a period of time.

Nevertheless, while some can get away not being laser focused sometimes, there are sports that the player needs to stay focused at all time.