The short answer is that they could lead to civil war.
The longer answer, if you have the patience for it, touches on what purpose democracy serves in a society and the warning signs that are flashing with increasing intensity but being ignored by one side of the aisle.
THE PURPOSE OF DEMOCRACY
Democracy serves as an alternative to violence. As Dr. Martin Luther King Jr. put it “Riots are the voice of the unheard”. People are more likely to accept an outcome they personally don’t support if they see they did get a chance to be heard but were outvoted by their neighbors. They may take a dim view of their neighbors but are more understanding of why the authority is imposing the thing they are.
By contrast, if an authority simply imposes a thing and the public has no nonviolent recourse to pour their opposition into, then some will resort to violent means to be heard. Democracy serves as a pressure relief valve. I have been clear that I think that making Democracy the highest authority is a dumb idea and prefer a Republic that has elements of democracy but I’m going to stay on topic. Democracy is a nonviolent pressure relief outlet to make voices heard.
WARNING SIGNS FLASHING
There are a large number of people who believe the 2020 election was stolen. Regardless of your personal views on the subject or of those people who believe as they do, even the blind cannot deny that such people exist. Those people do not believe democracy exists in our country. They believe there is voting but the votes aren’t counted fairly and so there isn’t democracy.
There are a number of agencies warning about the rising threat of right wing domestic terrorism. If there is any single issue that could possibly swell support for such a cause, it would be the belief that there is no real democracy and engaging in violence is the only way left to them to be heard. “[Violence] is the voice of the unheard”.
I don’t disagree with those who say Trump is mostly or entirely to blame for fostering this dangerous belief. He needs to go away and maybe go away to prison. It was reckless beyond words but here we are.
THE PROBLEM WITH DROP BOXES
The integrity of an election cannot be proved if there is no substantive verification of the legitimacy of every single vote. There has to be an auditable chain of custody from the voter all the way to those doing the counting and tabulation. Drop boxes break the chain of custody. There is no way to know who dropped a particular ballot into a particular drop box.
We have the technology to make things convenient, secure and highly auditable. Online banking does it. Crypto-currency does it. The idea that we must have an unmanned drop box that breaks chain-of-custody for purposes of convenience is ridiculous. The idea that we must have drop boxes when a sizable portion of the population doubts the integrity of the election and unmanned drop boxes make end-to-end audits impossible is foolish.
The Wisconsin laws predate Trump and the current mood of the country but they were put in place to ensure election integrity. That need is more pressing now than when the laws were originally written. The question you should be asking is why election boards aren’t using modern technology to allow you to vote using your smart phone in the middle of the night in your underwear, then logging back in 3 days later after new information has come to light to change your votes with all IP addresses logged and your front facing camera confirming you are you.