According to this author:
https://www.washingtonexaminer.com/opinion/the-air-force-wants-to-buy-more-f-15x-jets-and-its-a-huge-mistakeHe points out survivability, drag issues etc.... that would doom any F-15X in any foreseeable conflict, even "low threat" areas like Syria where the danger is Surface-to-air, owing to the lack of stealth.
At first I was nodding along with his analysis, but one thing he wrote made me question his conclusions.
That one thing is the F-15X would have capactiy of 22 air-to-air missiles. (Likely a similar complement of air-to-ground). From other reading, I see the F-35 has a capacity of something like 6-10.
The author never touches on this, but I recall another article about the F-35 retaining its stealth capabilities and using its sensors and targeting to guide missiles from another aircraft onto target.
As such, the F-15X's may be planned as a weapons platform for the F-35. Given the price I am reading in the article, it seems really high for such a purpose, but if the author missed this possibility, he is likely missing other issues as well.