I think it's common sense that we're not going to build starships any time soon. An NTR powered ramscoop? Please. I'd love to see how much fissionable material that would require.

"A practical magnetic field decellerator would cause structural failure and/or impossibly high magnetic fields."

That's an interesting statement. Care to elaborate?

"Nope, cosmic rays require 6 feet of concrete to shield against (not an option for a startship). DNA based life is incompatible with interstellar travel due to cosmic ray damage."

Or a significant increase in medical technology. Besides, this is a Johannes class misconception. Cosmic rays are significant in a high energy per particle sense, not a high flux of particles.

To counter your pessimism, maybe propellantless propulsion will someday be an available technology. That would change everything, don't you think? Im not wearing ruby slippers, just thinking that at times in the past there have been periods where it was believed that all there is to know was known. Do you really think we know everything now?