Your mistake (and theirs) is in viewing this through a traditional political lens.
Trump will claim victory no matter what happens.
His supporters don't care about the govt shutdown (many prefer it) and if you take him at his word, he's willing to keep it shutdown as long as it takes.
$5B on a $4T budget is a very small concession to make. Tactical error by the Dems in lowering themselves to Trump's level over a measly $5B. Their only chance is if he caves, which I doubt he will.
Also, no one's vote in 2020 will be changed by this. All this does is prevent the Dems from appearing above the fray, as the adults in the room. And at a certain point they may start to lose some of their more pragmatic supporters.