I’ve seen this first hand, we have a friend who has a genuine medical reason for DAS but she will also use it to skip a 15 minute queue, that she has no medical reason to skip.
We’ve done it ourselves, my GF is fine in line for 30 minutes before fainting even becomes a threat, but when a line is 25 minutes and all you have to do is click a button on an app to avoid that line it’s easy to abuse it.
It’s the soft abuse that is probably the biggest problem.
A third line, for leaving and rejoining your party, would be useful to not only DAS but also everyday guests who just need to run to the bathroom or grab a drink, unfortunately you are correct there’s no room for it in the majority of rides.
Not sure having people hold places solves the problem either, it wouldn’t be a very fun vacation to have dad standing in line alone all day while mom and the kids are waiting elsewhere, or the wife waiting in line alone while the disabled husband is sitting alone on a park bench elsewhere.
There’s no real one size fits all fix, a third line would definitely be helpful though.