If we cut existing beenfits, it still seems to me many disabled/unemployed people would be worse off with BI, or about the same, as the current system. So we're left with massively higher taxes, and people who can't work will be left poorer.
The second article mentions all the secondary social benefits, and all the costs we'll avoid associated with people living in poverty. I buy that, but a 18%-35% tax rate itself is also going to have a massive deadweight losses as well which he's not accounting for. Current marginal rates are 30-40%, so we'll now have 48%-75% marginal rates (or assuming a 35% increase, 65% to 75%). Think that won't make society poorer and decrease the incentive to work?
I want a stronger social safety net. But BI seems to me like a extremely costly way to do it. There are many things we can do that would have higher bang for the buck.
Here are two articles arguing that it's entirely affordable even at the level you suggest:
http://www.huffingtonpost.com/scott-santens/the-economist-ju...
https://medium.com/basic-income/universal-basic-income-as-th... (particularly interesting, IMO)