I doubt that Roger Ebert knew how he would feel in advance of his cancer and at each point as it progressed. This is a place he has come to.
We can't know ahead of time what will happen, how we will feel about it, and what decisions we will make about it.
A round of chemo to live another 30 years? Sure, I'll take it.
A surgery that might work, might not (actually no surgery is guranteed) so that I can move on? Sure.

We keep reinventing ourselves as we move through life. Who we are at 50 is not who we were at 18. With age and experiences behind us, have different priorities, different values, different definitions of joy, and what makes life meaningful. We are complex at any stage of life.

Death is certainly the biggest unknown in our lives. There are many scenarios: sudden, lingering illness, short illness. I don't think we can know what it will be like until we are there. So, besides setting up Advance Directives and other arrangements, it is something we can't figure out ahead of time. What a relief.