I would consider it likely that the 18th Amendment would remain in effect until after the Second World War. Its been mentioned above that US servicemen overseas would be exposed to alcohol, so I can see that the first Congress to be really made up of ex-GIs would move to overturn it. This is the generation that brought back pizza and italian food in a big way.
Also someone would notice that enforcement is just ridiculous and not very good. More importantly, that the government could get money from the controlled sales of alcohol. Someone will be realist.
Well, quite a lot of Vietnam vets were exposed to marijuana, heroin and other drugs while overseas, yet we don't see a large movement by Congressmen who were Vietnam vets to legalize these drugs.
And many people have noticed that enforcement efforts against the drugs we currently make illegal are not effective...they don't really reduce supply that much, and they create a huge criminal enterprise benefiting organized criminal gangs, just like Prohibition did in the 1920s. Yet, we haven't seen the "realists" come forward to legalize these drugs.
There's no particular reason...other than the fact that alcohol had an organized lobby funded by the beer and liquor industries, while marijuana and heroin didn't...why alcohol prohibition should have been any less long-lasting than the bans on these other drugs. If we assume a slightly more "puritannical" streak in the American population at large, the politicians might have decided to ignore this lobby and Prohibition might exist to this day.