Musar for Megillah 53:23

In the chapter "Bnei HaIr" (Megilah 27b): "Rabbi Zakkai was asked by his disciples: 'in virtue of what have you merited such long life?' He replied: Never in my life have I made water within four cubits of a place where prayer is said, nor have I called my fellow by a nickname, nor have I missed making Kiddush on the [Sabbath] day. I had an old mother who once sold her headdress so as to bring me [wine for] the Kiddush".

I believe that I can add another dimension to the understanding of this מדרש, and to relate it to the commandment of פריה ורביה, the duty to procreate. I also mean to explain why the Talmud (Megillah 27) says that the only reason one is allowed to sell a Torah scroll is to enable one to fulfill the commandment to get married and to beget children. The Talmud there simply says that the reason for this is the verse in Isaiah 45,18: "He did not create it to be a waste, but formed it for habitation." why does the Talmud not quote the commandment from the Torah which is very specific, and is content to quote an indirect reference from the prophets? The same question can be raised against a statement in Chagigah 2b, where the Talmud also denies a person who is partially a slave and partially free the right not to marry by citing the above verse in Isaiah as its reason. Tosaphot, in addressing themselves to that problem, come to the conclusion that the statement in Isaiah is more compelling than the commandment פרו ורבו. No explanation is offered why this should be so.