Even if it only took them 100 years to get to a star system 5 ly away, I would be amazed and impressed by a society that would be okay with investing in and working on at minimum a 105 year mission. Perhaps they live much much longer than we do :)
Cathedral building in the middle-ages was a multi-generational 80+ year affair and some of the Pyramids took even longer IIRC. So societies were/are definitely capable of working on ~100 year missions :)
Maybe a generation ship needs to have some sort of 'cathedral-in-the-sky' religion connotation in order for us to commit to building it. Sounds like an interesting idea for a sci-fi plot!