It isn't advantageous, or at least wouldn't be in the near term needed for the trait to be selected for without a host of other adaptations. There is a limited amount of space and resources, and older individuals crowding out the young wouldn't be very useful. Aging also wouldn't likely be completely absent, and the elder individuals might remain virile but have lower-quality gametes.
As to "how hard can it be to replace telomeres", directed evolution isn't really a thing. Wings could be useful to humans for example, but in order for them to work we'd need a faster metabolism, other sensory organs, stronger muscles, etc. These other traits might not be selected for, and thus flight is unlikely to evolve in an organism similar to us modern humans.