The F-15 first went into production when I was 19.  I am now 50.
Thirty-one years as our front-line fighter is enough.  It no longer represents cutting-edge technology.  The design of the F-14 is even older.
It is time for an upgrade.  From what I have read of it's capabilities, the F-18 "Super Hornet" is not an upgrade.
We cannot maintain our supremacy without spending the money.