Good question! I don’t think it’s usually possible to estimate that accurately.
That’s why I think it may make sense to play it safe, and just adopt a strategy of “doing our personal best”, while trying to promote other changes too (inspire others to do their best, push for policy changes, etc).
Of course I was referring specifically about people who, in your words, cannot do it. :)
I worded it as “we” instead of “some people” in order to take my fair bit of personal responsibility: Even though I fully acknowledge the incredible importance of Climate Change, through my actions I am often part of this group of irresponsible people I refer to.
That being said, I found your answer really enlightening. Thank you. :)