... and goes on to refute the claim that it's unnecessary for householders to practice in solitude.
I never stated that doing solitary retreats was not important. Of course it is important. I just question your definition of "sustained".
But we do not need to support monastic institutions for this purpose. Monasteries are hardly solitary, and the ratio of monks who actually do real retreat in monastic retreat centers to the number who are just there to do rituals and so on is very low. I am not saying "don't support monasteries" but I am pointing out that the reality of monastic life is quite different from the western fantasy of monastic life.