I agree with the sentiment. It's a lot healthier not to look at parts of your life as "phases" and more as a blend of rising and falling tides of success and happiness.

BUT. People don't define success as "forever" they define it as "until you pass away" it's a hard deadline and it makes perfect sense. If you are part of a marriage that you have a lot of fun and pleasure in. You don't want it to last forever. You want it to last 60 years or however long it will be until you die. If you love running a coffee shop, you would rightfully be devastated if operation costs rose and success ran dry before you were tired of it. And you don't want the success of the coffee shop to last forever, you want it to last at least 60 years, until you die.

Edit: to be clear cause I don't think it was, I encourage people not to have a set end goal for a hobby or a project. Success can be playing an instrument for a few years, or writing one book and dropping the hobby. But I think it's perfectly healthy to say "I want to be a writer for the rest of my life." And if you can't succeed at that and you have to drop your writing hobby, you have unequivocally failed at your goal. You just don't have to let failures like that define your life after the loss of your desired success.