Long-term unemployment is a big thing in Germany, and they are essentially getting a UBI-equivalent (generous housing including utilities, health care, TV etc, cash, ~1000€ a month in total on average). While it's hard to quantify "productive use of their lives", it's certainly not productive in a contributing-to-society kind of way, and there's also no creative explosion happening.
I believe the big misunderstanding is that intrinsically motivated people assume everybody is like them. But most people are not like them at all, so what might work for you will probably not work for them.
I expect so. Creating things are hard. Creating new things, even with little difference from the existing things, are also hard.
By the way, I'm curious about that. Won't people that have generous everything utilities will not just produce more babies? Or there is no correlation between UBI and population boom?
I believe the big misunderstanding is that intrinsically motivated people assume everybody is like them. But most people are not like them at all, so what might work for you will probably not work for them.