Massive is not a legal term.
The legal requirement is that you have to prove a sufficient level of fraud to call into question the election result.
In a real close election, say 10-50 votes, it doesn't take much fraud to steal an election. But in the presidential election, the closest state was around 10,000 votes. That is a massive amount of votes in relative terms, especially when you consider that most studies of election fraud suggests that the total amount of fraudulent votes nationally in a given election cycle is substantially less than 10,000 votes. Given that the best estimates place the level of fraud at about 0.0003% of the vote, you would expect to find less than 1,000 fraudulent votes nationally in the recent presidential election. (A typical voter fraud case involves the relatives of a politician claiming to reside in the right district when they actually live in the neighboring district. That might be enough if the family is large enough and the race is close enough, but is really stupid since most races aren't that close.)
As far as the purges of registration rolls, you are assuming that the voters were notified that they were being purged. If you don't get notice, the first time that you learn that you are no longer registered is when you show up to vote by which time it is too late to re-register.
Currently, ballots are counted by machines which are publicly tested before and after the election to assure that they are counted accurately. (Amazingly, even though the tests are publicly announced, nobody other than the designated Republican and Democrat representatives ever shows up, but millions will whine after the election that the machines must be crooked.)
When recounts occur, you have both Democrats and Republicans as part of the counting teams. Each member of the counting team gets to look at the ballot and tallies the votes. Then at the end of counting that precinct, they compare their numbers to see if they agree (which is recounts take so long and are so damn expensive).
The voter rolls are monitored as constantly as they can be. But, it takes time to get death certificates after a person dies and not everybody does proper change of address notices with the post office when they move. You can't adjust the voter rolls based on information that you do not have.