I am not so optimisitic about what life extension and other technologies that transhumanists get excited about would mean for humanity. I am reminded of Douglas Adams:

    "Meanwhile, the poor Babel fish, by effectively removing all
    barriers to communication between different races and cultures,
    has caused more and bloodier wars than anything else in the
    history of creation."

In that context many of these arguments seem moot. Even the idea that "More people make more progress faster" becomes speculative as "people" would no longer be constrained by genetics to specific distributions of neurons.

People tend to think that [current year] is super late and advanced in the history of the world, and seem to infer from that impression that things that haven't been solved yet are unlikely to be solved. The germ theory of disease is about 150 years old; practical uses for electricity are just over 100; powered flight is just over 100; not using horses as primary transportation is the same; etc. Now is early in the possible history of discovery. There remain hundreds of thousands of important things we don't understand, or even know about, yet.

Besides which, we are talking about a hypothetical future in which we have solved aging; to suppose that future would exist without comparably revolutionary advances in intimately connected biological problems like cancer and dementia is odd.

If you believe we're doggedly and linearly solving Problems Humanity Faces one at a time, and that techno-utopia is around the corner, I think that's mistaken. We continue to remain blind to unforeseen consequences, and it is unclear to me that we'll figure them all out before we wipe ourselves off of the face of the earth.

My second argument was that our current moment in time is not likely to be near the end of the possible tech tree, that large advances in technical knowledge and ability are very likely still in front of us, and that therefore it doesn't make sense to argue that if by now we haven't already solved [x,y] then we probably never will.

Your argument seems to be that we don't fully understand the unintended consequences of our inventions, and that there's a distinct possibility that some of those unintended consequences may cause us to go extinct before we get smart enough to stop making those kinds of errors.

I agree with that argument. I don't see any conflict between your point and my point - just between your point and some claim I didn't make about techno-utopia being just around the corner.

Some kind of "techno-utopia" does seem like a possible future, but not necessarily a probable one. I don't know if we'll make it there. We're not reasonable enough, maybe. And one of the ways in which we're unreasonable is that we tend to read into our opponents' arguments things they didn't say and impute to them beliefs we don't have evidence that they hold. That isn't helpful, and on the large scale it's distinctly harmful - it's a major part of the political polarization and divisiveness that is dragging down our collective ability to cooperate and come to reasonable compromises.