Why wouldn't he just let Watson help?
If Holmes allows the fight to continue, then the outcome is unpredictable. It may be that Moriarty may overcome them both.
If Holmes and Watson win and Moriarty is taken into custody, it is likely that Moriarty will continue his vengeful pursuit of Watson from prison.
Moriarty: Once we concluded... our business here
it important you know... I shall endeavour to find the most
creative of endings for the doctor, and his wife.
If Holmes and Watson win and Moriarty is killed, they may find themselves in prison as nothing has yet been proved against Moriarty to the satisfaction of the British state.
Mycroft Holmes: You can't understand the
delicacy the situation.
I passed the telegram on to my superiors,
but they were the ones who have brought Moriarty in
to advice on the peace process
in the first place. He has positioned himself brilliantly.
As one of our formost intellectuals,
he's a personal friend of the prime minister...
I believe you, but where's your evidence?
Watson: He's too good to leave evidence....
he doesn't leave loose ends.
How could he have survived this fall? Was this part of the plan or just good luck?
Holmes' use of the oxygen supply clearly indicates that he was intending to maximize his chance of survival. However, in the event of his own death, Holmes still wins
- Moriarty is dead, and Watson is safe from Moriarty's dreadful enmity.
Is it possible that Moriarty survived, too?
There is no information given in the movie as to whether Moriarty's body was found. His chances of survival are less than Holmes' as he was not planning the fall and was not prepared.
In summary, Moriarty can only be beaten by a sacrifice play
The narrative of the movie, based around a chess game, is that Holmes wins by making daring sacrifice plays that Moriarty cannot read and does not expect.
Moriarty: I... think you've just lost your most valuable piece.
Holmes: A winning strategy sometimes necessitates sacrifice.
Holmes throwing himself and Moriarty over the edge is the final sacrifice play that wins the game.