The decadal did prioritise two probe missions, which will have a cost of about a billion each. But realistically it would be one per decade, with the first launching around 2035. It's a limitation of the decadal system in that proposals compete in terms of scientific priorities, not cost or efficiency. So the smaller concepts struggle to compete. There are no fixed cap opportunities like there are for smaller Explorer missions. ESA has a better system in this regard, they have comparable M class missions where the best concept which fits within the cost cap wins. Euclid, Plato and Ariel are all ESA M class astronomy missions expected to launch this decade.