I don't think you'd be compromising at all by using said bikepacking pack for non-bikepacking uses. At the size/cargo you're talking, you'll have plenty of capacity. I'd say don't even think about pedestrian uses if you're going 20L and up. If you question it at all, take your contents to the store to see how it fits. You're going to try them on before purchase anyway, eh?
Well I haven't had much luck finding them in local stores, may have to make the long trek out to REI to check if they have em.