Chiming in as a former-ish member of the demographic you are just making stuff up about. There is no way to determine how much and how UBI would impact hikikomori because the demographic is inherently adverse to study.

I personally know that some crucial open source work is maintained by people with schizoid-avoidant spectrum issues. I know a lot of them but I won't out them here. hikikomori are driven to be invisible because their extreme pathological avoidance of attention. You don't know them and their contributions because they don't want you to know that they still live at home, out of their car door dashing because no company ever hires them, are shut-in because of serious unhealed trauma, are still deeply in poverty in such a wealthy industry etc.

A lot of these humans if given a no pressure handout of cash would likely contribute more to society. Would most not contribute? idk. But I do know that the contributions of those who would might offset all the others.

Many prominent pseudonymous devs have had hikikomori traits. _why practically inspired a generation of Ruby devs. visualidiot (RIP) was a crucial driver behind a lot of web dev culture in the 2010s. Heck, I made significant contributions to Joomla and WP themes back in the day -- you have probably used sites with themes or plugins I made. Also I ran a blog a decade ago that used to rank prominently in google and receive dozens of emails a month from people struggling with mental illness -- many people crediting me with saving their lives. Surely that is something of value to society.

Don't go around spreading bullshit like it is facts about a group of people we know little about.