Earlier quoted context omitted.
The concern for the GOP isn’t that democrats favor voting by mail, it’s that the GOP has worked hard to make it as hard as possible to vote in person in targeted (eg non Republican) communities. Widespread vote from mail defeats that as it means that their selective polling closures, and arbitrary barriers to voting in person, go away
Out of curiosity, what are those barriers?
The simplest one is just selectively closing polling places, or the related practice of requiring local jurisdictions to fund their own elections, which selectively impacts poorer cities or counties. Next comes voter identification laws which selectively target people who are less likely to have government identification cards (black voters are four times more likely to lack both a drivers license and a US passport compared to white voters). Finally you simply throw ballots from the disfavored precincts directly in the trash. County officials can do this in several ways, by not delivering working machines or official ballots to certain precincts, by forcing precincts to vote on provisional ballots that officials have the authority to ignore, and by subjectively discarding mailed ballots because the signature "doesn't match" according to a non-reproducible process.