The F-15EX is not "substantially more expensive than the F-35 to buy" - in fact it about the same as the F-35A at $80M flyaway (November 2020 - Boeing) and therefore cheaper than both the F-35B and F-35C.
The F-15EX is also much cheaper to operate than the F-35 at $27K per hour compared to $35K+ for the F-35.
The F-15EX is ideal for what it is being purchased for - currently recapitalization of ANG units in the homeland AD role - and obviously capable of a lot more if enough are bought to start replacing the oldest F-15Es.
"To Chief of Staff Gen. Charles Q. Brown Jr., it’s not a matter of one or the other. “It’s capability and capacity,” he said during a Defense One online event in October. While Brown insisted, “We still value the F-35,” he called the F-15EX an “opportunity.” Because foreign customers have invested heavily to modernize the F-15, the Air Force can leverage those investments and acquire an airplane that’s as good as a fourth-generation airplane can be, without laying out big dollars for development or tooling, Brown asserted."
https://www.airforcemag.com/article/...differentiator.