I think the evidence will show that people who are less attached or committed or motivated to vote, the more they perceive a barrier in front of them, which could be simply their lifestyle at the moment. They're too busy, have kids to take care of, or schoolwork or other things. This is just normal. Then, if you add a barrier about identification that is beyond what is considered reasonable, the danger, of course, is that people will simply self-select themselves out of the process.
A way to get around this, of course, is what you are studying, and it seems to me you're looking at it in quite good detail: the impacts of vouching versus other measures. I encourage you to continue to look at this question to see what kind of solution you can come up with.
The broader issue is that we have to work harder as a country to try to raise the motivation, raise the civic literacy, and raise the civic knowledge of young people. The danger isn't just at that one moment. Research shows there's a danger of habitual non-voting as people move through their life cycle, if you will. Your voting interests and your political interests change as you grow older, as you get a job, as you have kids, as you worry about different things. The danger will be that this youth downward trend that we've had will continue into other age cohorts as we move along, and then you're going to have a generational effect.
The ongoing motivational piece is important as much as—this is really my point—the identification issues that you are studying so assiduously.