Stock buybacks are a money sink. You don't do them when you could invest in your company. It's clear in Niji's money spending that Riku doesn't know what he's doing as a CEO.
You shouldn't sit on hundreds of millions like you're a fantasy dragon. You're supposed to re-invest. There's so many things he could've decided to invest into. Tell me, even if the stocks didn't crash after buyback, what would he have gained from it ? A few millions a year later ? If he had built his own 3D studio, for example, he could've easily made that much in a few months. What about setting up their own merch infrastructure ? Or starting offices in other countries to make collab projects easier for talents ?