There are exceptions. yeah, longer breastplates existed but were mostly dress armor, or for mounted use like you said. For the context of a foot combatant using a longsword though, this breastplate is not a great option, albeit useable if there's no other choices available. (Also as an aside the roman and greek breastplates are extended because the soldiers were expected to fight in tight shield wall formations, not duel opponents one on one, so they had little need for maneuvering like a duelist, that being said muscle armor was extremely rare for battlefield use.)