It wasn't like anyone better was going to sully their reputation by jumping onto that sinking ship, knowing full well that they would be scapegoated for failure and removed by the new board.
Koeman is being well compensated for taking a fall.
Xavi may wait until more of the financial dust settles. If he walks through the door now, it's likely to end like Lampard. That's not because he's at Lampard's level, but because the squad is a mess, the club hasn't the finances to fix it and the board is going to need someone to blame the next time elections roll around.
I'm also not convinced that Barca can actually afford to sack Koeman, unless Xavi is willing to work for peanuts.
Those problems can probably be contracted around - Barca could write Xavi an extremely long-term deal by today's standards that doesn't pay particularly well until year two once Koeman's wages are off the books.