Buffett is advocating that the rich pay more in taxes. Buchanan and the rest are ignoring that advocacy and trying to turn it into something about Buffett.
The issue with comments that amount to "just shut up and pay more if you want to" is that contributions to the upkeep of civil society shouldn't be based on the voluntary whims of the few. Governments need to be able to plan a budget, so they need stable, predictable incomes. You don't get that by passing the plate around every Sunday. You get that by having sensible tax rates.
Corporate taxes as a percentage of the economy and as a percentage of the federal budget have been falling for a long time. The rich have overwhelmingly benefited in the US for the last 20+ years while the middle-class has stagnated. There is nothing objectively controversial or radical about those facts or about Buffett's opinion piece.
FWIW.
Cheers,
Scott.