Are you forgetting how often European nations fell apart and had many civil wars and change of government forms? Don't you remember the Yugoslav Wars? Even the Kingdom of the Netherlands is younger than the United States. The USA has a continuous democracy since 1789
Usually a coup is counted as a success if the person executing it takes it remains in power.
Trump did not remain in power, and the attempt is counted as a failure.
It's a clear sign that there's danger to the democracy, but that doesn't mean the coup didn't fail or that the democracy has.
Hitler took power after a failed coup, but that doesn't mean that the coup wasn't a failure, since he didn't take power as a result of the coup.