It's an interesting proposal... and I can see both sides of the argument. It's a lot of money to spend on an aircraft type scheduled for phase-out, but given the development timelines and inevitable delays that will likely impact NGAD, that phase-out will likely be pushed back. I'm with Lonewolf, a Raptor in the hand is worth a lot more than NGAD promises in the bush. A third side of the argument would be "how many more F-35s could we buy with $9 billion and would that be a better use of our money".
Personally, I'd like to see a new production "F-22B" with the avionics/systems of the F-35, but that ship sailed a long time ago.