But is it a safe assumption that there were no "churches" in existence while Jesus was still preaching? What about people who responded positively to Jesus' message and yet didn't leave their homes to wander around with him? Wouldn't his ministry have resulted in clusters of disciples, or maybe even whole villages, that had committed themselves to Jesus as the messiah? (Richard Horsley agrees this is likely in his book Jesus and Empire.) And if so, did these people ever gather to discuss what they had received? Whether they gathered in the synagogue or elsewhere, wouldn't it be appropriate to consider these sedentary communities as a kind of "church"? And if such communities did exist, then why couldn't Jesus have had them in mind in those passages where he talks about how to conduct the communal life of the "church"? Sure, the term "church" itself may have been retrospected back by Matthew, but I'm not sure it's a safe assumption to think Jesus didn't plant stable communities of followers, or that his teachings would have never referred to such communities.
I guess what I'm saying is that it's very possible, even likely, that Jesus was a church-planter of sorts.