I wouldn’t add much to what Laurence Tribe has already said:
(1) Even if the Fourteenth Amendment mandates that interest on the debt be paid, there’ll be more than enough in revenue to pay the interest — the debt ceiling increase is needed to pay the other expenses as well.
(2) Even if the government didn’t have the money to pay interest (and, again, it does have the money), it doesn’t follow that the president may exceed his powers to do that, just as he can’t take property without due process in order to do that. That the president has a constitutional duty to do something doesn’t mean that he can violate other constitutional provisions in order to do it.