I hate that quote, like other quotes formulated by happy people who believe that everybody else is either happy in the exact same way, or else dysfunctional. For me, it is a blatant lie. To others, it is not, but the generalisation is still erroneous and untrue. The therapy relationship cannot be replicated elsewhere, so maybe it will be the same kind of lie, in the end. It is not possible for me to have that kind of conversation with people outside the therapist's office. I know I'm lucky to have the chance to do it now, for a few years, but I, too, wonder what it will be like afterwards. Hopefully therapy will help me get back to where I did not feel any need to confide in other people. It is how the lottery of genetics made me, after all. And I know that different people have different goals, and for me, the goal is not to be able to apply what I learn in therapy outside it, since that is, as the OP points out, largely impossible.

I don't think the solution is to stop thinking about important and deep subjects. I think reading is one answer: Camus, Kierkegaard and other existentialists for instance. Finding the answer to the existential loneliness.