> Warrantless border searches are literally older than the fourth amendment
That's not an argument for warrantless searches, though. A thing that's regulated is obviously older than the regulation. Killings are older than “thou shalt not kill.”
In many cases of constitutional law there's room for debate about what the founding fathers meant - what's with all those commas in the second amendment? What defines an "unusual" punishment? Is the commerce clause supposed to be as broad as Wickard v. Filburn makes it?
However, when looking at the specific question of whether the founding fathers meant for searches at the border to require a warrant or probable cause, we have clear evidence: The same people who wrote the Fourth Amendment also passed a law allowing customs officials to search boats at the border.
Of course, this doesn't preclude arguments on narrower grounds than border searches overall - I don't imagine the founding fathers imagined carrying naked pictures of their partners across the border, and border guards demanding a copy!
That's not an argument for warrantless searches, though. A thing that's regulated is obviously older than the regulation. Killings are older than “thou shalt not kill.”