My comment was rather tongue-in-cheek, but the MiG-25 was designed in the early 1960s when solid state was still in it's infancy and tubes were still the technology of choice for many applications, especially in some of the high power RF and radar circuits used in the MiGs.
As for the design of our own aircraft the changeover to solid sate devices and ultimately IC based circuitry was not driven by cost bit rather the desire for more advanced capabilities in terms of aircraft control, navigation and radar, communications and weapon systems. None of the advances systems in the current vintages of our military aircraft would be practical (or maybe even possible) using all tube designs because of size, weight and heat issues. I dare say the if the electronics of a current F-15 was built using all tubes you would have trouble fitting it all in a single aircraft hanger, never mind a single aircraft.