I wonder whether it would be possible to re-introduce the "I win" button, but in a way that would avoid the two problems mentioned earlier. For example:
1. "We Win": The idea here is that a spell might make the wizard good, but it makes another character better. It is not a new idea - even in 3E, there were some suggestions that knock should give a bonus to Open Lock checks, while invisibility should give a bonus to Hide checks. So, even though a wizard could cast these spells to open locks and sneak around if there was no rogue in the party, he would be better off if there was a rogue, and he used them to improve the rogue's ability instead.
2. The Narrative Win: Here, the "I win" button becomes a plot point, not a challenge. The wizard can, with a single spell, kill the BBEG in one round. However, before he can do that, he need to find the BBEG's true name. And he needs to find a rare component to power the spell. And he needs to fight through the BBEG's minions and henchmen before he can get close enough to kill him. And the party doesn't get any XP for killing the BBEG, except maybe XP for completing a quest. The lower the risk, the lower the reward, and in any case, they should have earned enough XP in the process of fulfilling all the conditions for casting the "I win" spell.