Well sure, Adams in particular maybe not, but for him to warn about this hypothetical scenario while it was ostensibly unfolding in front of his eyes is still something i can find a bit ridiculous.

> for him to warn about this hypothetical scenario while it was ostensibly unfolding in front of his eyes

Well why wouldn't he? See something, say something.

I'm sure he saw direct reasons to be worried about people becoming ungovernably immoral, and I bet slavery was one of those reasons -- after all, he did believe it to be a colossal evil.

So I'm not sure I understand your point.

Because he's talking about it as if it weren't happening. "Should the day ever come..." Well, sure seems like it already had

That's a misquote - that phrase doesn't appear in the letter. I don't think that idea was present either. Could you clarify which phrase(s) conveyed that to you?

Here's a link to the full text of the letter: https://founders.archives.gov/documents/Adams/99-02-02-3102

If anything, I think he felt compelled to write what he did (not only in this letter) because he felt a sense of imminent danger -- that if the people individually and collectively failed to rise to the high calling of good moral character, the new republic would not last. Remember, the longevity of the United States was by no means a foregone conclusion at the time -- the US Constitution had been ratified a mere 10 years before he wrote this letter, and Adams himself had just been elected as the second president the year before.

(All of which doesn't make his thoughts inapplicable to our time.)