I posted a link above where it went into detail the vulnerability the A-10 has to small mobile and man-portable missile systems. These warnings became evident in Desert Storm and only got more pronounced as this level of technology trickled down.
The A-10 was viewed as capable in the 70s and 80s, where your primary threats were guns and early mobile missile systems. And even then, the expectation was that the expected lifetime was not very long. They would delay and harass Soviet first-echelon forces, and pay a heavy cost for doing so.
That they took notable losses from a second-rate ground force that had been heavily attributed for 8 years against Iran told the Air Force that this day they would need to be retired would come sooner than expected. They were fine enough for the Balkans and Iraq in '03 when you could reasonably expect to eliminate most anti-air threats with the F-16, but this isn't expected to be feasible in the next conflict. Speed, sensors, and countermeasures (stealth and ECM) are what's needed. Low and slow won't be sufficient unfortunately. The A-10C got some of these improvements in the sensors and ECM department, but unfortunately it's not enough.
I'm not sure how the F-35 fits into this. One of the issues with the B-2's is that our fleet is tiny and they're super-expensive to run and carry limited ordinance. I suspect they may take some pressure off the B-2's with their mixed role.
They will be carrying cruise missiles too. Because $1m is quite a bit less than $100m (the plane).
They're all talking about sending in the F-35 first, but to me you actually send in cruise missiles and or drones first to take out the known threats, and then send in the F-35 to take out in-flight enemy aircraft.
Assuming you have half decent satellite intelligence (which we know the US does), it doesn't make sense for the US to risk pilot's lives no matter what they're flying.