The proportion between the two species in the equilibrium state will depend on the relative strengths of the two reactions - see the reaction coefficients. If dissociation had a coefficient of zero, it would be effectively gone, and the equilibrium state would consist 100% of dimers 0% of monomers. And similarly if it was all dissociation equilibrium would be at 100% momers.

The rate coefficients given above give a might higher strength to dimerization, and so we would expect equilibrium to consist _mostly_ of dimers.