The Russian Air Force would seem to pose a credible aerial threat to Ukraine.
Nah, our air force pose an overwhelming aerial threat. Nothing that Ukraine (or anyone else) could do to change such disbalance of power. So, basically - why bother?
Essentially, Ukraine have several possible threats:
* Russia - nothing could be done, so why bother?
* Donetsk separatists - did not possess any air force worth mention.
* Border conflict with Poland, Romania or Belarus - a low probability, but not impossible.