Here's a great example of how acquiring the F-35 lowers capability:
defense-aerospace.com/articles-view/release/3/158059/f_35-cost-reduces-korea-weapons-buy.html
The F-35 lowers capability because of it's price
Originally South Korea wanted an additional 60 upgraded F-15Ks with full weapons fit.
Then there was big blow up over the Koreans ordering an old generation "non- stealth" aircraft.
They changed the order to F-35 BUT:
- They could only afford 40 F-35As instead of 60 F-15K
- It now turns out they can't afford the required weapons or spare parts due to cost of actual F-35.
The F-35 may be a wonderful weapon system but it's so expensive it reduces capabilities by reducing numbers that can be acquired. Obviously it impacts on weapons and spare acquisition.
Lower numbers = greater susceptibility to attrition ala the regular accidents and failures that are common in military operations.
Overall it would impact other capabilities too – spending more than anticipated on F-35s means less dollars for frigates, transports, rifles, boots etc etc or schools, roads, hospitals.
The same has happened across Europe too – F-35 buys are being slashed by anywhere up to 60% due to aircraft not dropping in price as promised.
In the end Western airforces will be left with token high tech forces with no ability to actually do anything with them.
With the South Koreans, the F-15 more than exceeded North Korean threat levels (their main jets are Chinese MiG-19 knock offs (F-6), various MiG-21s and only a very small fleet of MiG-23s and MiG-29s).
The F-15 is still very capable against Chinese and Japanese too and will continue to be so in the medium to long term future.
But instead of maintaining aircraft numbers, spares capability and weapons, the South Koreans are going for a silver bullet F-35 fleet much like the rest of the world.