Companies that "do the legwork" of decrypting ransomware for the most part just pay the ransom on your behalf.

Presuming this results in a cryptosystem change for Akira, there’s a real number of victims who won’t get their data back as a result of this disclosure.

Whether the number is more than that of victims to date who can recreate this? Who knows

How would they get their data back if someone theoretically knows how to decrypt but never tells anyone.

I can’t remember the example (it was a conference talk a few years ago), but I’m pretty sure there’s LE and DFIR companies who also reverse this stuff and assist in recovery, they just don’t publish the actual flaws exploited to recover the data.

Key being generated insecurely is hacking crypto systems 101. The mere fact someone can reverse it probably means this is the first thing to check.

It was already disclosed to the bad guys that someone managed to break their encryption, when they didn't get paid and they saw that the customer had somehow managed to recover their data. That probably meant they might go looking for weaknesses, or modify their encryption, even without this note.

Other victims whose data were encrypted by the same malware (before any updates) could benefit from this disclosure to try to recover their data.