I'm not sure I understand the practical impact of this objection. Right now, massive life extension (routine 100+ yr lifespans) and solving serious permanent medical conditions (regrowing lost limbs, curing Down syndrome) are both significantly beyond our capacity. Any research into either topic easily qualifies as basic research and I would predict that increasing our knowledge towards curing permanent medical conditions would be useful in life extension, and vice versa. Put slightly differently, how would you expect our basic research funding to change if we abandoned research into massive life extension?

And even if Down Syndrome occurs in the same frequency after 1000 yr lifespans are common, isn't that an improvement for most people?