The argument of Torah decline during L Rebbe’s lifetime is not so pashut. Whatever are numbers of general Jewish population in the world, we can argue that post-WW2 time is when we saw the end of the decline that was occurring before. Just read Chofetz Chaim’s letters in Poland in 1910s-30s where he repeatedly calls on Yidden to stop sending children to anti-religious schools and have at least one kosher cheder in each city; to make sure that the person not only keeps shabbos himself but his children do; that many communities will not be able to hold by the government requirement of having one sanitary mikva per town …. And that was Poland. In US, R Nosson Sherman said tht during his parents time, the Rav would tell his elderly students to continue coming to the class because there might be none after them … (I recall R Sherman saying “class that his father was teaching” but wiki says that his father was storeowner, I may be misremembering).
It would be silly to argue which group is most responsible for the tremendous revival of Torah in the world last 50-70 years, nut Lubavitch was undoubtfully part of it, even as some of them might exaggerate their role.