Seems mostly like a stopgap to me. This is meant to replace a bunch of old F-15 C/Ds which are aging out of their operational life. The -18s are Navy, and so using them in the Air Force to replace existing -15s would be pricey on the logistics side, F-22s aren't being manufactured anymore and would far too expensive for the National Guard units these are going to, and the F-35s just aren't available in large enough numbers yet. The -15 has also turned into a very versatile aircraft, and it a lot cheaper to operate than a -35.

Part of the reason these old aircraft stick around is due to the very low risk involved with upgrades. We have -15s, we know they work very well, we know how to fix them, we know what to do with them.