With completely wireless earbuds, the rule is: when the battery fails, they have to be disposed of. Not so with the Fairbuds, that allow you to replace batteries in just a few seconds. Combined with a repairable design, the earbuds should therefore have an extremely long lifetime.
iPhone batteries are covered under warranty if they drop below - I think - 80% of original capacity. Using that as a benchmark, something between that and 50% is going to be frustrating for the average user. Perhaps frustrating enough to replace.
“Brick” caught me off guard too. When thinking about a product that can’t be used while simultaneously charging has a battery that’s nearly shot, though, it struck me as a fair description.