Old airplanes spend more time in the shop than new airplanes, and less out flying. This certainly isn't shocking.
It is important to procurement. An old C-5 or C-130 is in the air a third less often, or worse, than a new C-17 or new C-130. Two new planes equal three old planes. Also, maintenance isn't free.
So, while it definitely costs money to buy new planes, it is worth remembering that buying new planes also avoids costs and that they replace old planes on a more than 1-1 basis. Also, once you acknowledge that you have to buy new planes, the relevant consideration then becomes, do you buy expensive new units of old designs, or somewhat more expensive new units of new designs.
This doesn't mean that we should go hog wild buying planes. We have too many of some kinds, light ground based fighters like the F-16, and too few of others, like C-17s. The mix and total size of the air fleet should be adjusted. But, that doesn't mean that we can simply ignore modernization. While the nation may not need as many F-35As as requested, it needs some to replacing aging planes.