It's pointless to speculate about counterfactual situations. Wishing can't change anything. SOURCE: Matisoff, Blessings, Curses, Hopes, and Fears. I once heard a historian use this proverb when asked why the Jews didn't raise an army to fight Hitler. Variation:
.ווען די באָבע װאָלט געהאַט רעדער, װאָלט זי געװען אַן אָמניבוס
Ven di bobe volt gehat reder, volt zi geven an omnibus.
If grandma had wheels, she would be a bus.
SOURCE: Stutchkoff, Der Oytser fun der Yidisher Shprakh