Gee, that’s funny — we’ve had 100% mail-in voting here in Washington state for a couple decades now, and there’s been no substantial problems of which I’m aware.
Your opinion is really nothing more than a direct result of the policy pushed by Paul Weyrich, the founder of the Heritage Foundation. He said:
“I don’t want everybody to vote. Elections are not won by a majority of the people. They never have been from the beginning of our country and they are not now. As a matter of fact, our leverage in the elections quite candidly goes up as the voting populace goes down.”
That’s why the GOP has pushed so hard to prevent entire swathes of the population from voting, the most recent of which was the election in which the conservative-led SCOTUS — in accordance with the demands of Republican lawmakers — insisted that the election be held in-person in the middle of a pandemic (and yes, some were infected as a direct result).
You yourself may sincerely believe that this is all an effort to ensure all votes are legal and constitutional…but in reality, it’s simply a modern-day tactic that hearkens back to the days of poll taxes and literacy tests in the Deep South. On a side note, yes, those were mostly under Democrats…but the Democrats of the Deep South were also deeply conservative.
Then as now, conservatives are happy to defend the opportunity for eligible voters to vote…but only if they’re the voters that will vote the way they want them to vote.