People would still want money. UBI isn't going to be enough for a lavish lifestyle, people will still want to work. It would make lower-paying activities like art/open source more viable though.
Not really. The income tax didn't exist before 1913 and the government was way smaller so there wasn't a even need to collect a lot of taxes. Are you thinking of the 50s? Because even in that time people just avoided to declare all their income (it was easier back then) or used other loop holes to avoid paying taxes [1]
[1] https://taxfoundation.org/taxes-on-the-rich-1950s-not-high/
Not to mention a lot of other social programs that would be folded into UBI and eliminated overnight. Social security, food stamps, rent assistance.... That's not enough savings to pay for UBI, but it's an appreciable chunk.
And the existence of UBI would imply the existence of a national registry, which would drastically change the equations regarding illegal immigration. Border enforcement becomes a lot less important to some anti-immigrant demographics, if those immigrants cannot receive any welfare. Police for fraud, sure, but maybe you don't need as many fences and armed guards patrolling the border?
I'm just saying the knock-on effects of such a drastic shift in society would be huge, so expecting a simple comparison of current costs and revenues is so simplistic that it's guaranteed to be off by a lot.