“If it’s relegated too the backfield, a modified C-130 or C-5 could carry a lot more missiles.”
That’s true, but those can’t launch them at Mach 2, which gives them a LOT more range... Nor can they arrive where needed as quickly.
The same range issue applies to things like the small diameter bombs, which glide for tens of miles before impact.
OK, the B-1B can go supersonic, and carry a lot more than the F-15.
And I doubt the F-15 can pull Mach 2 with a full load under its wings.