Question about the Neutral Route

Say I was doing the genocide route up until one specific enemy who i then spared (meaning that i got the "but nobody came" message until i got to this enemy, e.g undyne) this enemy, then after sparing continued to use the genocide route tactic of shifting up and down to grind enemies, would this allow me to reach a level cap? And what would the ending (sans's call) look like? I was thinking of actually testing this out tbh, but i wanted to see if anyone else had beaten me to it.