Yiddish: If grandma had wheels, she would be a wagon.

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

אױב די באָבע װאָלט געהאַט רעדער, װאָלט זי געװען אַ װאָגן.

Oyb di bobe volt gehat reder, volt zi geven a vogn.

If grandma had wheels, she would be a wagon.