Why is that a problem? People's motivations and mentality change throughout their life. Understanding that there may be a lull of 3 years where someone is "just playing video games", and being OK with that is just being an empathetic human. They'll get tired of the game. They'll get tired of the same grind every day. They get tired of eating the same thing. They get tired of not having any goals. Things change. Our mentality goes through phases throughout our life, and we grow/evolve despite ourselves. Would you rather that person be a surly server at a restaurant, who doesn't want to be there, who only wants to be home playing games, and provides you with a terrible experience? And a manager with a terrible employee. And other employees being affected by the one person's bad attitude? Why NOT let them take advantage of the social safety net? To pursue their own personal development, or go to a professional that can help them get beyond their current malaise? AND be able to seek the help they need without the anxiety of having to hold down their job at the same time, find therapist appointments outside of their (variable) work schedule and traffic jam times, etc.? The customer at the fast food restaurant has a better experience, because the employee they interact with is there by choice. The manager and other employees at the restaurant have a better work environment and crew, and actually enjoy their work, rather than HAVING to do it just to survive. The game company makes money. The therapist makes money. The food delivery company makes money. And, in the end, the individual gets to be the architect of their own rise from the ashes. They get to feel a sense of accomplishment at overcoming their personal demon, rather than doing all the work just to get to neutral/zero/level again. Any demonization of people for USING their UBI is seriously stick-in-the-mud thinking... if I had UBI today, I'd cut my hours to 20 hours per week (because that's all the work I honestly have) and spend the rest of the time working on a sliding scale doing marketing projects for small businesses I love, but who can't afford to give me $100k/year. The idea of UBI is so mind-bending to Americans... we just can't even wrap our heads around what life would be like if we had ANY flexibility in our work at all... "I'm just praying that introducing a basic income doesn't lead to a lot of disenfranchised young people playing video games."
I think my issue is your focus on gaming. The exact same thing can be said for Mormons, or bow hunters, or Japanese high performance auto tuners, or pinstripers, or knitters, or writers. Your particular dislike for gaming (which you come by naturally due to your brother's experience), clouds what UBI would do for other people who laser focus on whatever their passion is. UBI would allow each of them to pursue the thing that they are most passionate about. A dedicated writer, working on their book, might get a job for 10 hours a week to supplement their UBI, and then spend 20 hours a day, locked in their room typing up their magnum opus. They would be perfectly happy to send off their manuscript via email to their editor, do all the editing and rewriting virtually, and never have to do a book tour - or leave their house - again. It's not about gaming/gamers. It's about where people WANT to spend their time and purse the thing they are passionate about. Will some people become shut-ins? Yeah. Sure. Absolutely. They do that today... UBI wouldn't change their core tendencies. And with a proper social/medical system in place to support people with clinical problems, UBI allows them to get the help they need, if the reason for their cloistered existence is a mental issue rather than a simple preference. It's a problem because gaming can be a very insular community and highly addictive.
There is definitely a balance between "I am comfortable never working" and "I am uncomfortable never working." There needs to be something appealing about working, and there needs to be something unappealing about not working. Balance in the United States is definitely skewed towards maximum (fatal) discomfort for not working. The real argument is that moving forward, the incentives for working won't go away but the opportunities will - so the tech college job keeping your brother out of LoL is much less likely to be available, and much more likely to be taken by people who want it more. There is already a massive group of mostly-men who play video games rather than contributing to society. That group is likely to grow. An entrepreneurial sort would figure out a way to tap that resource in such a way that you can profit off their labors because as a group, it's going to get bigger before it gets smaller, UBI or not.