and yet their hypothesis is true, there are already many people, with or without UBI, that volunteer, create things and in general help people surrounding them without any reward and they are the backbone of every society, not the career-chasers

I think phenomena like hikkikomori have more to do with (at least perceived) social rejection than lack of motivation. If the only acceptable message you receive from society is that you must chase the brass ring constantly and any setback means you are an abject failure, then withdrawing from the pain of that rejection makes sense for anyone who has experienced enough setbacks or strongly feels alien to that culture. A broader societal shift would occur if it was truly universally understood that everyone has value as a human being separate from their labor market leverage or capital accumulation.

There will always be strivers who measure their self worth against superficial standards (Russ Hanneman “doors go up” hand gesture here), I just don’t see why everyone should be forced to play that game or starve I suppose. Giving everyone the option to settle for a life of basic dignity while caring for those around them, or going all in on some academic / creative pursuit seems equally valid investments for society.