There's no need for that — Kusuke already has an on-screen revelation!
Kusuke had already resigned himself to not being able to challenge Kusuo any more after deleting his powers, because it would be "one-sided bullying".
It's extremely sad and disappointing for Kusuke because messing with Kusuo is pretty much the only fun he ever has, even though he knows that Kusuo hates these competitions. That's why he always has to force Kusuo to play, either through the fake limiter, or by setting up some other consequence, like taking his friends hostage or threatening to make his robotic double streak naked at school.
The cat tank was Kusuke's attempt to win against Kusuo once and for all before fulfilling Kusuo's wish for deleting his powers. Since studying Kusuo's powers is basically Kusuke's life work, it kinda seems like he planned on coasting on that victory for the rest of his life hsfjdlshfks
But since Kusuke lost again, he has to resign himself to never having won and never getting another chance to win. Not very fulfilling, and now he's going to have no hobbies any more.
Which is why Kusuo's offer, to keep playing a little longer, has such an impact on Kusuke! Kusuo NEVER volunteers to play, he's always either responding to Kusuke's challenge or has to be forced into it! And Kusuo making this offer shows that he understands that Kusuke actually made a huge sacrifice in giving Saiki the power deleter, because it essentially means giving up his life's purpose.
It's an incredibly kind gesture, inspired by Akechi's genuine expression of love and kindness in testing his abilities against Kusuo's — not to win and assert dominance, but to check whether he presents enough of a challenge to be fun to play against, because Akechi just wants to be best friends who have fun together again.
Kusuo hasn't gotten to have fun in games since childhood probably! Every single one has been pretty much life or death for the last decade (including time loops), and even when he's not battling Kusuke he has to carefully fake mediocrity at school sports or risk everything.
But when playing with Akechi, the only stakes at play are a slice of cake, and Akechi doesn't even gloat about his win or claim superiority. It shows Kusuo what playing CAN be like, and so he's able to turn around and share that idea with Kusuke instead.
And for Kusuke to be presented with the chance to just play without having to figure out how to coerce Kusuo into participating each time, this is huge! Of course, it's a limited time offer since Kusuo does plan to delete his powers soon, but the fact that Kusuo offered at all is an incredible reversal of a lifetime of animosity.
And that's why Kusuke is chill and cooperative for the rest of the series. He did his big final blow-out evil gambit (though he lost), got a little more playtime then he had resigned himself to, and his relationship with his brother is better than ever. In fact, Kusuo actually trusts him enough to ask for his help with the volcano (after spending 6 years in the loop).
In response, Kusuke moves back home to supervise Kusuo while he removes his powers and seems to be genuine in his advice to help Kusuo stay "powerless". Of course Kusuke can't help but be excited when Kusuo's powers return though, that's literally of his enrichment ever. But STILL Kusuke tries to come up with an alternative solution for the meteor based on what he has on hand to give Kusuo a chance to live "powerless" a little longer.
Anyways, it's shown only subtly in canon but Kusuke and Kusuo really turn their whole 20+ year rivalry around as soon as Akechi shows Kusuo that kindness and new way to play, and it's really so powerful when you think about it!!
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