Firstly, the proposal to allow the markers to split is a much better proposal because it would force teams to not just keep attacking down a narrow corridor. Even if you reduce the number of players, it isn't going to force a more lateral game. What it will do is make the dummy half run more of an enticing option. Think about it. The forwards would be more fatigued and the gaps up the middle will be more.
The second issue is the sin bin/send off. At present a team must have 9 on the field or they forfeit. I can see a scenario that this could happen if they reduce to 11 players. Do we really want a game to finish in one team forfeiting?
Lastly, the defence and the determination of teams in goal line defence will be lost. I guarantee that any team the gets a full set in the oppositions 20 will almost always score (9 times out of 10).