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The F15EX will carry equipment intended originally for the so called Golden Eagles a number of F15C and Ds that had been intended to serve out to 2040. However 10% of the F15C and D fleet are grounded and 75% have flown beyond intended life cycle already leading to the new buys. It’s the same rationale that justifies the bulk of the Russian Airforce buys of SU35 and MiG 35.
The Russians are buying the Su-35 and MiG-35 because they didn't have any other choice really.
They designed the Su-57 with the intent it will replace their high-end fighters. Not complement them.
They designed the Su-57 to be easily serviceable like their current fighters. They do not apply RAM mats on the surface of the aircraft and have easy to access panels like older aircraft. This is part of the reason why US 5th gen aircraft are so expensive to service.
But mass production of the main production Su-57 type might be 5 years away.