What are the dangers in a democracy of allowing votes to be counted later and later after election day? What dangers are there?

Nothing was done in 2020 that was not done before. 

Oh you mean the states that have done that for YEARS AND YEARS? There's nothing new here! The only thing new was the number of mail in ballots. More than 20 states have laws long on their books to accept mail in ballots post marked on election day. 

The danger is that it might actually learn which candidate received more votes instead having to guess based on the ballots it could count in one day.

There are none.  Zero. 
Do you not understand what 
a 'post mark' is ?   The count is being done by human beings, it takes time.  All votes up until the specific cut off time are VALID.  Slow counting humans is why it takes days.  If you're American how do you NOT know this?

The only issue would be if they weren't finished by the deadline for the Electoral College to meet - in mid December.
We're not remotely approaching that.

A much greater danger is if votes are left uncounted because of an arbitrary cutoff.
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Re: your "hypothetical" - That's why ballot counting is done in the presence of bipartisan observers.

What you describe can't actually happen.  Pretending it's anything like what's going on right now is pathetic BS.

You're not trying to pass off pathetic BS as reality, are you?

Troi, a whistleblower was told by a supervisor to stamp predate on ballots. I can't imagine what sentence that draws. The govt. Postal System is VERY protective of it's rules, procedures, integrity on many facets. That dude isn't talking to local cops, he's talking to agents at The Bureau and nobody's crackin jokes.Think about what somebody can get for mail fraud. If you invent something, you can package your prototype mail it to yourself leave it unopened, if a dispute leads to a courtroom challenge you can enter that package as evidence and that stamped date on it is above reproach. For I'm sure good reasons and future consideration there are just parts of USPS  laws and punishment that are hardcore. Ok, went on tangent. Kinda same for Electorate, lotta hard laws. You never want that count to go overnight or chain of custody can be challenged, but if security is in and parties have  no discrepancies it's fine.bottom line is if there are no reports to investigate, all must be fine as both parties have right to observe