There are plenty of ways to find meaning without giving people a "be productive or die" ultimatum.
And not everyone is industrious or contentious enough to work for such reason or work because they want to.
This has nothing to do with capitalism but with human nature. You go down that route and it'll only spur people to demand more from the UBI in the same way that the War on Poverty is ever increasing in scope despite the fact that poverty today has almost nothing in relation to poverty a century or more ago.
The basic necessities of survival are not simply a driving force behind things like meaning and worth in life, they are THE driving force behind them. All of this is the problem modern man is facing where we've become so successful in the struggle to survive the bottoms fallen out beneath us and everything driving us to live tooth and nail will not prevent us from limiting ourselves.
I certain know that through the times of privation I've endured. As much as I can look back and recognize the good within them and the satisfaction of getting even simple things like a new toothbrush they are not something I'd willingly return to because of how hard things were then.