The messianism flows naturally. Nasi Doreinu + no successor = Moshiach.
The Frierdike Rebbe had messianism on a much smaller scale, but it still existed, because he had no sons, and there were those who felt he had no successor.
RMMS became rebbe on 10 Shevat 1951. At the time he was almost 49 and had no children. It was pretty much a given from day one that there would be no successor.