> If there's something I can (want to and capable of) work on that's 100x more valuable than what I'm doing now, why isn't somebody hiring me to do that now?
That's like asking why 2000 years ago the farmers didn't just buy a tractor and a combine and do their farming that way, since that would have generated so much more value. The technology doesn't exist right now, we're talking about a scenario where it will.
> Similarly, if some AI is capable of writing better software than I do now, I don't get to choose another job that pays 100x. It's just that my current job will cost less
Correct, assuming that it's only writing software that gets automated, and nothing else. Society as a whole will be wealthier by virtue of being able to afford more software, but not 100x wealthier. But if 99% of everything gets automated, the jobs that remain (and the new ones that get created as a side-effect) will indeed pay 100x more in terms of real purchasing power. Whether you think of that as "this job is 100x more valuable" or "everything else costs 99% less" is just a matter of perspective.