vote by mail (and similar ballot harvesting, bulk ballot dropoffs with hazy chain-of-custody as from a nursing homes and immigrant communities) are new, based on paper, and open to abuse.
It's not where we were.
traditional absentee balloting was a small scale thing used by college students, military personnel, etc. and if it was messed up, it was not likely to change outcomes or a threat to counting accurately (no election is perfect)