The U.S. Air Force is much larger and has many more planes at its command. That means that it can field more specialized aircraft. F-35s and F-22s can support ground forces near enemy air defenses and go after missile sites and other fighters while A-10s or the proposed arsenal plane attack ground forces from behind the F-22 and F-35 shield.
This isn’t to say that the Air Force is necessarily wrong to divest out of the A-10 to bolster the F-35. The Warthog can’t stay on the battlefield forever. But if the A-10 has served its entire career in the post-Yom Kippur world, it seems like a shallow argument to say that it couldn’t possibly fight and win for another 5 or 10 years after nearly 40 successful ones.