I think when companies found out that they had an issue with their Payroll software calculations, they mostly tried to solve it as quickly as possible, to put a line in the sand - from that point onwards at least, no additional errors were being made. But they still had many years in the past, of issues which needed to be fixed.

I think what you're proposing probably would have worked for reducing the communication issues in the future for any employees who left after that. I didn't hear of anyone who did that, but that definitely doesn't mean it didn't happen. Likely no one thought of it because I would guess most people didn't expect it to take as long as it did to fix. That the people who left while the recalculation was going on would just be a few more compared to everyone who had left in the previous 7 or 10 or 15 years (I think different companies came to different opinions for the time period they needed to retrospectively fix).

It's a shame that the bureaucrats / politicians / voters who are responsible for these hard-to-comply-with rules will never bear the costs.