I agree with your post, except to say that I recall the performance of the Dark Souls games (certainly the original that I saw) to be utterly, utterly, utterly dreadful. I recall and underground swamp at something like -5 fps and with updates at such irregular intervals it made the Andes look like a sine wave. So good enough for the dev could be abysmal and nauseating by real people terms.
I just watched DF's update look at version 1.03 of DS3, and the framerate is actually pretty steady at 30fps, its apparently an issue with frame pacing that gives the stutter. Hopefully the developer works this out when they release the Trilogy. Your absolutely correct though, what the developer considers to be acceptable performance could be anything playable. Who knows, maybe they think 20fps is adequate.
Its also difficult to say how developers will utilize the extra performance when docked. I believe Nintendo's intention is for games to render 720p when portable, and the extra performance when docked makes 1080p rendering possible. I'm sure this will be the case for the majority of Nintendo's first party games, but there isn't really anything to stop developers from targeting 720p when docked, and either reducing resolution to sub HD or accepting poor performance when in portable mode.