"you could also just add the basis to this year's tax return and use the basis going forward."
That doesn't work. The tax code does not permit picking and choosing which years to apply the basis.
Failure to file Form 8606 showing the application of the basis does not mean that the distributions did not include a proportional amount of basis, it just means that an erroneous tax return was filed. To determine the amount of basis currently remaining in the traditional IRAs, one must prepare all of the missing intervening Forms 8606 even though the tax returns for years prior to 2021 cannot be amended to claim a refund due to being closed tax years.
