You're conflating "efficiency" with "disenfranchising voters."
Mail-in voting enabled citizens who otherwise simply couldn't vote, to vote. Citizens who, more often than not, were from already disadvantaged backgrounds.
Ok, now compare other things they do to enable voting. Is voting day a holiday and/or do workers get guaranteed paid time to vote? Are the ballot boxes equally accessible for most people, and not strategically placed to disenfranchise certain voters?
Mail-in voting enabled citizens who otherwise simply couldn't vote, to vote. Citizens who, more often than not, were from already disadvantaged backgrounds.