Using your metaphor, safety net...
Safety net is in place to catch people when they fall.
Should people really be on 'safety nets' permanently?
If your healthy reasonably smart adult children just wish to live off of you and play games forever..., yeah, it's certainly nice to allow them the safe environment so that they won't starve or get into trouble. I have nothing against THAT point of your argument. Heck, your adults kids might have a very generous trust fund made available to them! However, my point is that being that NICE won't solve the real problems with people who refuse to get up.
People have to learn to stand on their own feet. Even if you're born missing some limbs, find crutches, wheel chairs, robo-limbs... we don't have to just lay on the net for the rest of our lives. UBI will cause some people to never learn.
People need to learn to aim higher than safety net. Again, nothing against safety nets, just that we can't allow people to stay on it forever. It's just unsafe for everyone.