I am finding myself pulled to the idea that objective probabilities must always go back to objective tendencies. If so, then on pain of vicious regress, we must have a metaphysically necessary being (prima facie, perhaps an aggregate of multiple beings) which has objective tendencies that ultimately ground all objective probability. Such a being cannot be merely abstract as it is causally efficacious.
Moreover, I am inclined to think rational subjective probabilities are our attempt at modeling the relevant objective probabilities in the face of our ignorance. If so, then if one denies such a first cause, one cannot coherently engage in probabilistic reasoning about reality.
A lot of work is needed to work out a cosmological argument along these lines. I don't have it worked out. It's just an intuition based on a lot of thought about probability theory.