Are you seriously contending that someone who describes themselves as Democratic-affiliated is not more likely to vote for the Democrats than the Republicans?
Someone who is a non-voter isn't voting for either by definition. If that person becomes a voter, their political circumstance has changed, practically the only thing we know about them is that their political circumstance has changed, it is an unbased assumption to presume their pre-existing opinions are preserved across that change. The very cause of that change could be the changing opinion.
There are some situations where the logic checks out: if non-home owners are disproportionately non-white, and you increase the percentage of people who own a home, the natural expectation is that the percentage of home-owners becomes less white. That is not a certainty, there are still questions of how the increase in home ownership came about that could make that not be the case, but it's a reasonable expectation, in large part because people aren't changing their race.
There are some situations where the logic does not check out at all: if the non-religiously affiliated disproportionately don't believe in God, and the percentage of people who are religious increases, it is not the natural expectation that the percentage of religious people who believe in god goes down. If they still didn't believe in God, they wouldn't have converted, converts are more likely to have serious faith in God than the people who were just in a religion already. In this case, people are explicitly changing their opinions, it would be unreasonable to expect their pre-existing opinions to carry over.
Funny enough for a conversation on this site, the most directly comparable analogy I can think of is video game opinions. Imagine polling that said that the disproportionate majority of people who don't have a gaming system preferred PlayStation. If there is a sudden increase in the percentage of people who own gaming systems, should we expect the market share of PlayStation to increase? There is one element of logic that would expect that to be what they are buying because that's what they previously said they preferred. There is another element of logic that would say that their previous preference was not enough to convince them to buy a console. When they said they preferred PlayStation, they weren't buying at all, so clearly their opinions have been changed by something. If that something is the release of the Nintendo Wii, it's not gonna benefit PlayStation.
You are considering this like the first possibility, you are seeing voting like its this thing everybody wants to do but some people are locked out of, and if you let them it, they're still exactly the same except they are voting. But people aren't locked out of voting, it's actually very easy, unless you go to a busy precinct specifically when everyone gets out of work, and then its roughly as difficult as riding a rollercoaster. Most people who don't vote are just choosing not to, and it is silly to assume their opinions remain the same when suddenly they choose to start voting.
Let's look at the same statistic from two different perspectives:
1) You are seeing the idea that people who don't vote are more likely to support Democrats, so if more of them voted, we expect be greater vote share for Democrats.
But there's also this:
2) People who support Democrats are less likely to vote, so if more people are voting, we can expect the population to be less supportive of Democrats.
When they weren't voting, they supported Democrats, but that also means that when they previous supported Democrats, they weren't voting. What political engagement got them to vote is important, and the question you need to answer is why you would expect the outcome of that engagement to be different than the existing population of voters. If there were a trend in election results demonstrating Democrats doing better in high turnout elections, you'd have your case to support your assumption, but that doesn't exist. Data suggests that an expanded pool of voters tends to follow the same voting pattern as the pre-existing pool of voters, it seems to me the most reasonable assumption given all we know is that people engaging in politics are more alike than the people who aren't, regardless of how long they've engaged in it.