If the previous generation already had a decent quality-of-life, there's less reason to assume that the next generation will have it even better.

I do agree that the rate of improvement may be lower for already highly developed countries. And that the difficulties solved in developing countries may have a much larger positive impact then small steps in more developed countries.

But there's no artificial upper limit to quality-of-life. You wont hit a barrier and be forced to stagnate and go down: there's so many issues in today's societies that need solving.