Speaking as an ex-Apple employee, I'll just point out that a really malicious actor could have used this to harm some significant percentage of the installed iOS infrastructure, and done critical damage to Apple as a company with it. In fact, I don't know the percentage of users still on <12.3, but maybe they still could. A band-aid fix for this one bug should not be where they stop here.
or...
"Apple can turn off imessage to users with OS < 12.3 until users update"