Sevens is a game mostly played by women or under fifteens. There are some adult men's teams,but they mostly focus on rehabilitation in hospitals.
Now that 7s is an Olympic sport and players have a serious chance of making a living at it, the tendency is to specialize in either that form of the game or 15s. It's also tough to devote the time needed to 15s practise if you are busy working with your 7s side. There's no actual rule against participating in both, but the fact of the matter is that if you are going to succeed in 7s, you have to devote your time to that particular form of rugby. Nor is a high-level 15s team likely to give a regular player the time off to train for 7s. I'm afraid that the day's of backs and loose forwards freely moving from one version of the game to another are pretty much over. You were never going to see either McCaw or Read playing in the Olympic 7s anyway. 7s is a game which emphasizes pure speed, and neither has the pace to play it at that high a level.
