It is, but it's also important not to alienate the person you're trying to help.
I'm not being sarcastic - you're absolutely right - but it's very easy to come across as spouting happy-clappy cliches ("it gets better!"), or claiming to know their life situation better than they do.
Depressed people aren't always right about their prospects, but they aren't always wrong, either.
It is important to realize that other people in this world may have lived longer, gained more experience, and understand more than you do.
It's also important to realize that a huge part of depression therapy is trying to convince people they can be "cured" or can recover, when evidence points to that being highly unlikely - all the actual studies suggest that it causes permanent, irrevocable changes. And the author of the very book we're discussing specifically expresses skepticism about the efficacy of therapy.
It doesn't always get better. In fact, most times it doesn't.
I have 0 experience of your life. What I do know is that if there's a small chance that I'd never have to suffer another major depressive episode again, I'd consider that good information to have. Even when the odds aren't in my favor, I consciously make the choice to focus on the possibility. This isn't Pollyanna, it's practicality -- thoughts impact mood. Part of CBT is learning about your thinking, and one of the things that helped me get into a pattern of recognizing my own thoughts was to separate observations, suspicions and conclusions.
I appreciate your trying to help, and I don't mean that in any kind of a condescending or patronizing way. I shouldn't have been so quick to disregard what you're saying. And it's certainly true that variations of "binary thinking" are a hallmark of depression - although I guess what you're describing isn't quite binary thinking even if I think it comes from the same place, an absolute certainty of negative aspects of something and bottomless doubt of anything positive, or anything else at all about it. I'm glad you found CBT helpful to you and I wish you luck and happiness in your future.
It is important to separate what we know from what we suspect.