I think this is where your analysis breaks down. And where the difference between "strict rules to cover everything" and "loose rules to help DMs make judgement" comes into play for this edition.
Does it make sense that you can peek from behind cover and attack from hiding? Yes of course it does. But does it also make sense that if you do this, you might be spotted and thus your hidden benefits get cancelled out? Yes that also makes sense. OK, so what do the rules say for issues where you should have a chance of doing something, but your chance might be spoiled due to circumstances? They say you apply disadvantage to the check (in this case, a check to remain hidden while attacking).
Which is exactly what Mearls said when asked about attacking from a place where the circumstances say your target might know where you are.
That is a ruling supported by the rules. It's just that people got used to looking for a specific rule covering every specific event, that they're not so used to looking to general rules to help make a judgement for specific events not called out in the rules.
Right, same sort of circumstances as peeking around cover to fire. You have a higher risk of being discovered than normal, because your opponent is more likely to be aware of your location - either because you just shot at them, or because you just peeked your head out from behind cover. Seems right to me.