How large are the various groups of potential voters?

[Garrett78]

3. Yeah, the problem in 2016 was one of turnout. It wasn't so much...

...that some Obama voters voted for Trump but that millions of Obama voters didn't turn out for Clinton (including hundreds of thousands across key states).

Like you implied and as is made evident in the 538 piece (The Moderate Middle Is A Myth) that I linked to in the OP, there isn't some mass of astute middle-of-the-roaders with a clearly-defined moderate ideology who will vote in accordance with whichever party puts forth the candidate closest to the center. The potential for the greatest gains are among youth, POC and white suburban women. As well as 3rd party voters and non-voters. It's true that the strongest indicator of not voting in the future is not having voted in the past, but just a fraction of the non-voting population (numbering more than 100 million in 2016) could make a huge difference.