If you let delegates from FL and MI sit, you're punishing states that abided by DNC recommendations and held their elections/caucuses later.

And, you're really telling candidates that they should ignore the DNC and campaign in all primaries, whether authorized or not.

A good way to end up with a completely disorganized mess where no one listens to the DNC.

A do-over election in FL/MI has been proposed, and seems to be the most reasonable way forward.