See I see this differently.
As has already been said, several states have gone to largely or entirely voting by mail due to efficiency, convenience and the positive effect it has on voter turnout. Making it easier to vote predictably leads to more people voting.
I don't view expanding vote-by-mail during a pandemic as incompatible with having a normal election. I view it as giving people more options to exercise their constitutional right, safely, however they see fit.
I think the GOP is concerned about vote-by-mail because they know if more people vote it will be bad for them. It's why there have been so many undemocratic voter suppression policies in so many red and purple states since 2010. In my experience speaking with Trump voters when I do voter outreach, I certainly hear them use arguments about voter fraud, stealing the election, "If you can go to the grocery store in person, you can vote in person," etc. I know those are the defenses that Trump and the GOP use. But I think most people know why they're really concerned about more people voting and it has nothing to do with election integrity. It has to do with power.