I think you have your regrets mixed up. I can’t think of three people who had that regret.
Riki had the crash and let it control his life.
Rin couldn’t make friends and relied on her brother.
Kyousuke didn’t want to leave them behind.
Masato didn’t know what the point in life was.
Kengo didn’t allow himself to have fun.
Komari couldn’t accept death, and regretted everything leading up to death.
Kud had no place in the world. She just abandoned.
Yuiko didn’t get to experience any strong emotions.
Mio forgot her first friend, the imaginary Midori.
Haruka had nowhere to feel safe or loved.
Riki and Rin not being strong enough was the prime goal for the people who made the world. Not because of a regret, but because of a wish. The world was also born from a mix of goodwill and regret, but Riki and Rin were definitely the biggest aspect for the majority of the dreamers. The world was powered by regrets, so those regrets (including Riki and Rin’s) had to be cleared for the world to end. Riki and Rin becoming strong was just the preferred outcome, and luckily enough, their regrets were solved as a side effect to becoming stronger.
Riki and Rin may not have been a part of the group who created the world, but it doesn’t change the fact that the world will remain as long as they regret.
Essentially, Riki and Rin’s future was the catalyst. The regrets were the fuel. Riki and Rin’s future, and everyone’s regrets, are separate things. They just coincide very well.
