Gatorade for athletes.
> this doesn't imply that there should be no limit.
Who is even claiming these things. :/
I don't know what this means. Are you referring to the manual that came with human existence or are you referring to what the FDA says?
To address your analogy, researchers are in a sense writing the Missing Manual for Humans, as our species didn't come with one.
You're free to accept or refute these recommendations, but I personally am inclined to put at least some trust into their collective efforts. Or if you trust different recommendations more so than these, I'd love to see them.
This mentality coupled with incorrect recommendations helped the trend of "fat free" foods that would be high in sugar, not that long ago either.
I have to say I find your pushback at the term "official" perplexing. Why do you disagree with this descriptor? They're literally official. It doesn't get more official than this. Sure, they're sometimes wrong, as your fat-free reference suggests, but that's beside the point.
I can't find a specific source for a ceiling of 5 g/day, but it's pretty uncontroversial that more added sugar is never better. The lower you set your maximum intake, the harder it is to maintain, since sugar is added to so many foods.
5 g/day is a somewhat arbitrary cutoff that's lower than official recommended maximums, but this doesn't imply that there should be no limit. Pick one. Or start with "as little sugar as practical," log how much sugar you end up consuming under this plan (using a food diary, food scale, etc.), and adjust as needed.
[1] http://sugarscience.ucsf.edu/the-growing-concern-of-overcons...