Blaming puritanical opposition to fun seems like a 'just so' narrative that fits some of the apparent facts, but is contradicted by others. For instance, American culture glorifies caffeine, particularly coca cola which is portrayed in corporate propaganda as a 'fun' drink. When it comes to commercial products like that, there seems to be no hint of an anti-fun bias, ...except perhaps in the Mormons.
While anti-fun attitudes probably play some role, I think it mostly comes down to a pro-productivity mindset. Caffeine is generally understood to enhance productivity, and for that reason may as well be a revered god in the American pantheon. Alcohol is borderline; it's a known quantity that generally seems to leave productivity intact except in cases of alcoholism (alcoholism is consequently seen as shameful). Alcohol bans have been attempted, and are still intact today in remote regions of Alaska where the productivity impairment alcohol induces is seen as a particularly concerning matter. Cannabis remains maligned because it's seen as making people lazy and content, not because it might be fun. And amphetamines are legalized in circumstances where doctors say it will cure productivity impairing disorders.
The Prohibition was driven primarily by organizations like the Methodist Church and the Anti-Saloon League which were explicit about their religious convictions so it's hard to argue that it's just a "just so" story.
Prohibition was also advanced by suffragettes who were tired of being beaten by drunk men. The truth is multifaceted. As I said, anti-fun was probably part of it, but taking that kernel of truth as a simple narrative is misguided.
Furthermore anti-fun is not the sole religious motivation in play. Puritanism / American protestantism is also associated with the glorification of productivity that I describe above.
That fact provides a crucial context. It's easy to come away with the narrative that Prohibition failed so people just gave up. It also coincides with women achieving a lot more political and power, which gave them more options for getting away from abusive alcoholics. That played at least some part in the end of the drive to eliminate alcohol.
You bring another good angle, but you also seem to dismiss the OPs seemingly good angle as well. In my opinion, both of those are probably a part of it, there's a lot of people in a country, not everyone will justify their feelings the same way. It's very possible some are opposed due to puritanism, and some are due to impact on social productivity, and some on both, and some others on completely other basis.
We could have some survey based study that tries to ask respondent questions whose answer could maybe begin to give us a little understanding of how much each of these angle might be at play in a given population. That would already avoid a lot of this "I think" "You think" debate.
I think alcohol fits into the puritanical framework pretty well actually. hangovers are built-in retribution for people who consume to excess. I suspect puritan types find it particularly upsetting when drugs do not punish their users the next morning.
I suppose, though alcohol seems most fun when consumed in moderation, which avoids the hangover, avoids (major) productivity impairment, and is encouraged by our culture.
While anti-fun attitudes probably play some role, I think it mostly comes down to a pro-productivity mindset. Caffeine is generally understood to enhance productivity, and for that reason may as well be a revered god in the American pantheon. Alcohol is borderline; it's a known quantity that generally seems to leave productivity intact except in cases of alcoholism (alcoholism is consequently seen as shameful). Alcohol bans have been attempted, and are still intact today in remote regions of Alaska where the productivity impairment alcohol induces is seen as a particularly concerning matter. Cannabis remains maligned because it's seen as making people lazy and content, not because it might be fun. And amphetamines are legalized in circumstances where doctors say it will cure productivity impairing disorders.