Ever since the 19th Century and early 20th Century discoveries of the nature of electromagnetic radiation and subatomic particle physics, the fact that particle and electromagnetic radiation in space is deadly to potential space travelers and even deadlier at speeds that are a significant fraction of the speed of light has been well known. So your concern represents a strawman argument insofar as it may pretend there had ever been a serious proposal for humans to attempt to accelerate a space craft to those high speeds. In reality, the only serous proposals seek to a means of transiting interstellar distances using relatively slow speed spacecraft over periods of generations or discover a means of transiting those interstellar distances within fractions of a year or number of years by utilizing the effects of quantum mechanics to bypass normal spacetime and its relativistic restrictions.
I was addressing an issue which i believe most of those whom i have heard repeating newscast about habitual planets are not aware of, but that they assume this generation can get there. And it is not only radiation that is a problem, but hitting even a minute object can be deadly at extreme speeds (shields are down Captain!).
using relatively slow speed spacecraft over periods of generations or discover a means of transiting those interstellar distances within fractions of a year or number of years by utilizing the effects of quantum mechanics to bypass normal spacetime and its relativistic restrictions.
Unless you can get the Amish to be space family Robinsons, i think the second might have more chance of success!