No, you're quite right. FU money in 18 year is what will happen if nothing else changes in my life - and it's a better outcome than the unplanned alternative, which is chained to a job and broke in 18 years. But there are lots of other alternatives. I hope to get married and have a family, and my partner will probably contribute as much as I do or more to our shared savings. We might come up with some new goals together. But whenever I meet that person, I hope to bring more than debt to the relationship.
So I don't think it's as simple as "get FU money then stop" because you really do need to think hard about what you hope to earn in your life, to what end, and how it trades off with other great things in your life.