[RadioAstronomer]

There still is the problem of the energy/resource requirement to fund, build and power such a craft. Not to mention the sustainability of a complex construct such as a "generations ship". You can only carry so many spare parts or raw material to replace worn out machinery. What happens if your perfectly tuned ecosystem evolves in a way that it is no longer viable?

[exmarine]

There is another problem - space debris. Over thousands of years, speace debris would destroy any spacecraft on such a long journey. Space is not just an empty vacuum.