"Come back tomorrow" only works if that wizard has that spell in their personal spellbook. Spell Versatility makes it work for any sorcerer of the appropriate level because they all have all the sorcerer spells potentially available to them tomorrow. Visiting one sorcerer is therefore the same as visiting any other sorcerer and there is no gamble. Visiting a wizard on the other hand might not be useful.
We are talking about an NPC right?
In which case... they will either have the spell or they won't. Period.
If the DM says that the sorcerer doesn't have access to that spell, no amount of players pointing to optional rules will change that.
I think that is a big thing that is a difference. Either between playstyles or intentions. If my players go to an arcanist of any stripe, I determine whether or not that NPC is capable of doing what they want. If they go to a wizard looking for help reaching a location and I decide that Wizard casts Wind Walk on them, even though that is not a wizard spell and the wizard should have no way to cast that, then that wizard casts Wind Walk on them.
So, I don't get the argument on how this rule changes the interactions with NPCs. It just doesn't, because the DM has always decided if a given spellcaster has the necessary spell. And they will use the same method to determine that that they always have. And if they change that, it is their choice as a DM.