A few things I've thought about after the true pacifist ending:
papyrus, being so important, likely now never wears the suit of armor
Undyne probably needs to be in constant reach of water
There is likely a KKK-esque group that still hates monsters for being their enemies so long ago or thinks they're freaks (especially the amalgamates)
If Flowey didn't escape (you see him tunneling throughout the game) then if any tales of him spread, which they surely have, someone would go back in and murder him. If he did escape, there is no way he is not trying to absorb human souls to turn into PS Flowey or Asriel again.
Sans is happy but is constantly reminded of, and scared of, the chance of a reset
Alphys has much information and will likely be forced to help human scientists if she resists
Because killing a monster immediately turns it into dust, most may have never eaten meat, the only meat being available being rats and spiders as far as we have seen. Some might be disgusted about the idea of killing another living thing to eat it
Comps and videos like these make me want to give up my efforts to beat sans ; just go, watch a video of it and what the pacifist route looks like afterward and do a true pacifist one more time and leave it that way forever. Especially that last one.
Get to the point where you would slice him. Reset. Watch ending on YouTube (it's pretty much only like a 5 min cutscene, so if you feel this way you won't lose much). Do pacifist. Watch good time videos with awareness that Papyrus was right in the end.
Not sure I'm getting what you mean here? Are you saying the pacifist ending/route still changes if you almost kill sans but don't at the last possible moment?
What I'm saying is that you can enjoy your bad time with Sans (after all it would be pretty stupid to do genocide without it's main attraction) and just stop the progres before the route culmination (so no, it doesn't **** up your game yet). The part about not ending the fight was here only so you wouldn't feel bad for killing Sans.