My own father passed roughly a year ago as well, also at a particularly advanced age. I had a similar experience, where I became frustrated with the lack of what I perceived to be care. As in "give a shit", or even a desire to find an actual diagnosis. One doctor told me, as my father lay intubated a foot away, that [the Dr's] best gift he gave his own parents was to DNR them so they never spent a day in a nursing home.

A year later I've realized that my father was 80 years old. We all have to die at some point. Your father was 87. I understand you're upset. It may be that there was more for the medical system to do, there may not have been.

At what point does spending possibly millions of dollars to extend some one's life 1 year, 2 years stop becoming good and turns into actual harm?

They have to go at some point. Even if it is someone else's fault.