I don’t think they had to convince anyone.
What are the numbers in any given year for a Presidential Election? 120,000,000 or so
2019 had 160,000,000? Not buying that.
The Democrats knew, out of pure hatred for Pres Trump, that they had about 60-70,000,000 votes. They fabricated the rest with Mail In Voting, ballot harvesting and, most importantly, enthusiastic help from 5 or 6 state governments. Could not have gotten away with it, knowing what happened and how it happened, what states have taken action to eliminate Mail In Voting, Drop Boxes and ballot harvesting? None that I know of.
I think they had to convince some, and they did it over time. First, they portrayed Trump in the most negative ways possible, starting with Russian collusion, then impeachment. I know a few people who actually were sitting on the fence, because the economy was pretty good under Trump, but all they heard was how good Joe would be and those mean tweets had to go! These are the same people who are now regretting their vote. Do they realize they’ve been taken? I’m not sure. They believe so much of what they are told on the boob tube.
As you said though, few if any states have taken steps to make elections more trustworthy.