It's not firmware - the issue develops very gradually rather than all at once. This is part of what makes it such an insidious fault, like the boiling frog analogy.
Make no mistake, the issue is in no way resolved on new manufactured 1st gen pros. New manufactured stock are more likely to be covered by the original purchase warranty and so there is less pressure on Apple to extend support.
I recently had my pre-Oct 2020 pros replaced with new manufactured stock under this support program. Within less than a month the replacements developed the same sound issues, supported by Apple's diagnosis and another free replacement.
I don't think I use them in an unusual way - I don't even wear them during exercise. There is just something inherently flawed in the design that causes the sound quality to subtly degrade by a significant amount.