Jet engines and airframes require constant maintenance. Constant use requires more maintenance.
The more advanced they are, the more stuff breaks down, the more maintenance they require.
That is both time consuming and costly..................
Correct. Modern Russian military aircraft are known for relatively limited service life. In contrast, US and NATO military aircraft have much longer service lives and are intensely maintained and flown frequently, even in peacetime.
I suspect Russia also has the ability to build NEW airplanes