That IS a great possibility. The other side of the coin is that it takes much longer for mail-in ballots to be counted and it could be used to prevent the current President, should he win, from being acknowledged as such.All states have and are using mail-in ballots to a greater or lesser extent, and both Pence and Trump used them in 2018. My wife & I have used them for around 20 years now here in Michigan.
What I believe Trump & Co are likely up to is to be able to call the 2020 "fraudulent" if he loses, and this could create an extremely nasty situation nationally.
But that wasn't my point. There are different laws for different states. Some states have in their laws that anyone can present themselves at a voting location, sign up, register and vote all in the same moment. One can vote in NY, run across the state line and vote again because of that law.
A non-conservative Politico (a commentary) simply notes the reality of an expanded mail-in ballot can open a litigation can of worms (I prefer early voting - and even a national voting holiday)
Opinion | Why Vote-by-Mail Could be a Legal Nightmare in November
I think it is a balanced review.