veraca is right, all of the paper badges that I saw had holographic stickers (although I never paid that much attention to other people's badges, unless I was trying to figure out what character they were), we even had a holographic sticker on the plastic badges one year. I'm not sure what kind of sticker they have though and depending on what kind of stickers, those too can be faked. And that's something that I can see affecting how quickly event/panel lines will move, since now the staff are going to have to be looking at everything closer to make sure they're legitimate.
My group has preregistered since the first time Kumoricon opened up the ability to do so. I've watched the lines over the years and I've seen the difference between the prereg and the regular lines. In the end, there isn't that much difference between them. The biggest difference was who the staff member was and how good (and efficient) they were at checking the people in. I remember the registration and check-in lines we had to deal with in some of the past years when they had to print out or photocopy items for us. Those were some of the worst lines I've ever had to deal with, and we were in the preregistration line, not that it seemed to matter due to everyone having to wait on the printers. Which is why the idea of "print on demand" for everyone scares the crap out of me.
Not only that, but having the paper badges instead of the nice plastic ones is going to be a pain. There have been numerous times when I, or one of my friends had reason to be grateful that our badges weren't paper, especially my daughter's when she was teething and we couldn't keep her from putting everything in her mouth. We taped hers shut so she couldn't pull it out, but she was really good at fishing mine out of my holder when I wasn't looking...
I've seen people lose badges into the sink while washing hands. I've seen people knock over beverages into their badge holders while eating. I've seen them fill up with rain. I've seen people forget about them and go swimming. Yes, I'm sure that in the grand total number of attendees over the years those times may, or may not be significant. But to the person who now has to go get another badge and deal with more registration lines it will mean quite a bit.
I don't work for the con, or know the state of their affairs. They may have had their card printer(s) break and did a cost analysis and found that it would cost them more to fix/replace it than it was worth and that if would be better in the long run to switch to only paper. Or they might have just looked at the overall costs and decided that a paper only route was the cheapest method and gone with that, I don't know.
What I do know is that, to me, paper badges look and feel cheap, especially compared to the high quality plastic ones we've had for many years. I know that there are those of us who would preregister for the souvenir card badges. It may not be the only reason, but it is something that certainly factors into the decision making. My group remembers the first year that they had the card badges. I can't count how many people with paper badges saw them and expressed regret that they couldn't have one as well. That was a year when we weren't in well known cosplays, and yet there were so many people envious of our plastic badges that I lost track of how many people I had to tell about why I had a plastic one and not a paper one. I even had someone ask me if I was a guest speaker lol.
Not only are there the people who liked the plastic badges, or people like my group who like the card badges and getting low numbers, but there are definitely people out there who preregister for reasons besides just the potential for faster lines or there wouldn't be an auction each year for badge number 1. Even if I had to pay a $1 surcharge for the privilege of a generic plastic event badge (with hopefully a place to write my character name and in place of a paper one), I'd do it, since I know it would be worth it, just for the piece of mind knowing that I don't have to stress out about being hyper-vigilant over the environment my badge is in for fear of it getting ruined or damaged.
I know ultimately that it is the con's decision, but I greatly hope that they will reconsider going paper only and at least give us the option to upgrade to a card badge, even if we all get the same one, with no customization options.