Letters like this accomplish little. In fact, someone here just had her alcoholic explode in anger while reading a similar letter.
Presumably you have said all of this before. You really don't need to spell out your conditions for staying. If he hasn't made a move to this point, it's unlikely he will have a sudden realization that he wants sobriety. If he does anything, it will be to placate you and get you off his back, and you will simply be prolonging the inevitable.
If you've had enough, I'd suggest you start making some concrete plans in that direction. Have you talked to a lawyer? That might be a good first step to find out what your options, rights, and obligations might be if you leave.