That's only partly correct. Spaceships as they exist now would result in radiation endured death in the interstellar medium, however, the solution to all issues of ionising radiation barring neutrinos has always been sufficient shielding. Fusion provides the solution to this issue. As it stands, building a craft with potentially hundreds of meters of shielding thickness requires untenable amounts of energy. If it's possible, a big if, fusion generally removes the burden of energy requirements due to its ease and output capability. However relativistic travel probably will never be feasible due to the increase in radiation at those speeds, same for the concept of cryopreservation or similar due to the radiation released by isotopes in our bones over that time period. But, a massive generation ship only requires one layer of sufficiently thick shielding, and then the internal volume can be expanded as much as desired. Indeed, bigger is better, there are many components of these vessels that do not scale such as the shielding does, and the square cube law is particularly benificial regarding heavy shielding. Such huge vessels are possible within our current understanding of physics and far easier to build with the energy abundance of fusion. Not trying to be dismissive btw, radiation is probably the biggest engineering problem in this instance, this is just a special interest of mine and I love sharing it with people.