Thing is... that 1/8th of the capacity that is "dirty"... just how dirty is dirty?Perhaps around a 8:1 ratio, fresh to dirty?
I mean, if you routinely run your oil into the chunky stage, then yeah, 8:1 doesn't sound all that good.
If you change the oil and it's however you want to think of it--half spent, 3/4 spent, something less than optimal--then it's more like 16:1, maybe 32:1, maybe better even.
And of course, even if you removed every drop of oil each time, giving the engine insides a hose down with brake cleaner, technically the last mile you drive for any given oil change, the oil is in the worst shape that it'll ever be in for that last mile--it's so bad and worn out, that's why you're changing it, right?
[For the record I usually walk away for 5 minutes or so, it's still dripping when I come back but I don't care, the oil wasn't "plumb wore out" and whatever's left would have been good enough for another 5k.]