1. Destroy the viability of the post office as a business (a long running campaign)
2. Disenfranchise voters (a relatively recent campaign)
Tens of thousands of people were disenfranchised during the primaries because their ballots were received too late, even if they were postmarked on time. [1]
Tax returns are considered to be submitted on the date of their postmark, but ballots are not. There seems to be an important disconnect here tied to the different incentives for the state.
Dropping ballots off at dropbox makes a lot of sense, but it won't work for everyone. The rules also vary and availability is often poorly communicated. Plus, most people won't expect to have a problem with their ballot being counted until they find out that it wasn't counted.
But the problem doesn't start with submitting your ballot. First you have to successfully register to vote-by-mail (some states require a form to be mailed in), and most importantly you have to actually receive your ballot in time. This also happened during the primaries, and so people had to go vote in person after all (or be disenfranchised). [2]
Lots of things we take for granted will stop working with a failed postal service, but voting is particularly key right now. It is the only path towards attempting to fix the issue at hand.
[1] https://www.npr.org/2020/07/13/889751095/signed-sealed-undel...
[2] https://time.com/5874571/new-york-state-general-election/