It also increases turnout, something that Democrats always support and the the GOP always opposes.
Even if we assume mail-in ballots were susceptible to fraud, why would we think only Democrats would perpetrate that fraud? Why would it be only a way for Democrats to cheat?
In reality Trump just wants the chaos, to be able to accuse his opponents of fraud, based on nothing, all as part of his attempt to de-legitimize his impending defeat.