Complex animals nonetheless have a method of producing young cells in their offspring - which capability is obviously a requirement for gene survival. That we don't have the same sort of capability to refresh the cells in our own mature bodies is either an accident[1], or the costs/risks[2] of it aren't worth it for our own benefit, or (per Lander, I guess) is counterproductive to your genes compared to short aging-enforced reproductive generations.

(your argument is good and I agree with it)

[1] (more complex design simply hasn't been reached yet, or it's impractical for adults, requiring a womb, etc.)

[2] I have no idea what risks would be present in a world where we replenish ourselves with young cells, but I can imagine at least novel types of bad growth and rejuvenation, aside from the normal cancers