If they make glasses free 3d theatres they might have to give out glasses that make it 2d cause I heard 3d can be bad for kids vision. That’s why they made the 2ds and marketed it for kids.
Anything can be bad if you don’t take a break, even staring at a computer monitor all day. I fail to see how it could possibly cause more eyestrain than a flat film. Unless they go with a goofy high frame rate or something. We had plenty of 3D stuff when I was a kid, from Viewmaster to moving 3D lenticular photos. Nobody ended up cross eyed or anything.
Also, the universe around us is in three dimensions to begin with. If you’re stuck in the second or fourth dimensions, I don’t know what to tell you. 😉
From what I have read, 3d movies can cause eye strain due to unatural constant change in depth and focus, due to all the scene changes. But a well made 3d film that takes that into consideration should be just fine