> More people make more progress faster.
Really? Have you ever seen an urban slum, or an overpopulated rural area?
> Stop having kids
Even if this were a viable solution (history teaches us it is not) dont you think that this wouldnot deprive many from most of the meaningful aspects of their lives?
> If living longer comes with too many disadvantages, we'll know 100 years from now and decide then.
What if longevity causes social changes which are difficult or impossible to reverse? What about those who will suffer the many disadvantages?
> Old people suck.
No, they don't. Whoever makes this argument does not deserver a response. What is true however is that most old people seem to have less flexibility to adapting to change, and with accelerating progress the chasm between the generation widens. The generational conflicts may be very high in the ageless society you envision.