I think we should not assume that our current understanding of physics is complete, as there are known gaps and major contradictions, and no unifying theory yet.
Thus, there is some chance that future discoveries will allow us to do things that are currently considered impossible. Not only computationally impossible but also physically impossible (like it was “physically impossible” to slow down time, until we discovered relativity).
The hypothetical future capabilities may or may not include ways to retrieve arbitrary information from the distant past (like the chronoscope of science fiction), and may or may not include ways to do astronomical-scale calculations in finite time (like enumerating 10^10^10 possible minds).
While I agree with you that much of the described speculations are currently not in the realm of possibility, I think it’s worth exploring them. Perhaps there is a chance.
Not sure about infinite, but assigning a massive value to lives saved should be the way to go. Say, $10 billion per life.
Imagine a society where people actually strongly care about lives saved, and it is reflected in the governmental policies. In such a society, cryonics and life extension technologies would be much more developed.
On a related note, “S-risk” is mostly a harmful idea that should be discarded from ethical calculations. One should not value any amount of suffering over saved lives.