Temporarily taking away some of his abilities, for only a day isn't really comparable with forcing a complete change of attitude and behaviour for the character. An Oathbreaker isn't likely to contribute to the party much.
I think you are conflating a mechanical change with how the player plays their character. Mechanically changing to an Oathbreaker doesn't require or even suggest that the player suddenly play their character's personality as drastically different.
Additionally, I disagree with taking away their abilities as a punishment, especially just for a day. If it is only a day, is that really even meaningful? Especially if the players happen to have some downtime following the encounter? On the other hand if the player has to go an entire adventuring day without their abilities during a time when the party is facing a number of obstacles, you are essentially asking that player to not participate in combat, at least not in any meaningful or impactful way. That seems too punitive. How is that going to impact the player's fun, and how is that going to impact the fun of the other players if they have to use more resources to carry the weight of one of their own while they are handicapped? There are any number of ways to create meaningful and fun consequences that further the story without requiring any kind of drastic change. And yes, I'm including changing to an Oathbreaker as well as removing their access to their abilities.
And yes, I would say that an Oathbreaker paladin is much more able to contribute to a party than a character without any access to their abilities. I think you are assuming that a player mechanically becoming an Oathbreaker suddenly becomes an entirely different person, one that is only working towards self-interest and personal power. But an Oathbreaker paladin can be a paladin that struggles wondering why their powers have changed and how this might be a reflection of the darkening of their soul. It doesn't have to require a drastic personality change, but instead can be more subtle or nuanced depending on how the player would interpret such a change, were they to agree to such a change in the first place.
Additionally, this seems pretty relevant because I am currently playing an Aasimar Paladin in a game. He has been the epitome of a paladin. He gives away all his treasure to the poor and donates to the church he is a part of, he is always using his healing on any NPC he comes across who is injured, keeps his opponents alive rather than slaughtering them in combat, and works tirelessly to do the right thing even when it is difficult. In fact, it's become kind of a running joke in the group that my character gives away all the good stuff and uses all his powers on the NPCs to the point that there's nothing left when the group needs it (don't worry, they are ok with it and have just as much fun in the challenge this creates). But my character also faced a super difficult choice. Free a bound devil and give up his soul, for knowledge of the dungeon they are trapped in that can help him and my character's companions survive. At first he scoffed at it. The only reason he didn't kill the demon was because he was bound, and killing it would lead to its freedom (even if it meant returning to the 9 Hells as a lemure. But after the party encountered the deadly creatures and traps deeper in the dungeon, and knowing they were also working against the clock, the paladin made a tough choice. He sacrificed his own soul, reasoning that he was a soldier and thus expendable. If him sacrificing his soul lead to a greater good through the ability of his companions to survive, he reasoned it was worth it. There is some other context, with him encountering other paladins of his order that had fallen (thus believing it was inevitable) and having experienced a crushing failure in which he lost a super powerful holy relic to a cambion that absolutely played him (as well as a critical failure against a sleep effect, but still a cool surprise by the DM), so the choice is not
totally out of the blue. Still, after all this, my character is still trying to do good works and bring the light of his god to others, even if he may never again walk within that light. And now, he is an Oathbreaker, but he is still struggling with how he can do good and make the world better in spite of his fall.
) , then nothing he did actually violated his oath.
