While I support a universal income gradually reaching up to (and eventually perhaps well above) the level at which it could fairly be called a UBI, it is not at all the case that any UBI level up to the pre-UBI GDP per capita is affordable.

A universal income will affect behavior, and through that real GDP; at some levels, it may actually improve GDP by reducing job market friction and better allowing people to retrain, retool, and optimize their contribution and their own returns; OTOH, there's certainly a level at which it draws people out of the labor force in large enough quantities to overwhelm any advantage from reducing friction, crashing real, post-UBI GDP.