After carefully inspecting 290 Russian MiG-29 fighters, which were grounded for over two months, after one of them crashed last December 5th, over 30 percent were found to be suffering from metal corrosion. The cause of the December crash was structural failure (the tail separated, in flight, from the rest of the aircraft). It was initially believed that poor maintenance and a shortage of spare parts was the main cause for a string of similar incidents. But the inspections revealed corrosion problems in aircraft with as few as 150 hours in the air. This is still probably a maintenance (or lack of) issue, because even the low air-time MiG-29s have been in service for several years. The aircraft just sit there, rusting away.
The MiG-29 has been an increasingly troublesome aircraft, while the Su-27 (and its many derivatives) has flourished. But all Russian fighters have suffered from years of poor maintenance and lack of upgrades. Russian Air Force generals have warned that, without a large infusion of money for upgrades, they will lose more and more of these aircraft to old age over the next decade. Currently, Russia only has about 650 fighters, including the grounded MiG-29s.
The MiG-29 entered Russian service in 1983, as the answer to the American F-16. Some 1,600 MiG-29s have been produced so far, with about 900 of them exported. The 22 ton aircraft is roughly comparable to the F-16, but it depends a lot on which version of either aircraft you are talking about. Russia is making a lot of money upgrading MiG-29s, mainly for export customers. Not just adding new electronics, but also making the airframe more robust. The MiG-29 was originally rated at 2,500 total flight hours. At that time (early 80s), Russia expected MiG-29s to fly about a hundred or so hours a year. But some export customers flew them at nearly twice that rate, and now Russia is offering to spiff up those airframes so that the aircraft can fly up to 4,000 hours, with more life extensions upgrades promised. This won't be easy, as the MiG-29 has a history of unreliability and premature breakdowns (both mechanical and electronic). Compared to Western aircraft, like the F-16, the MiG-29 is available for action about two thirds as much. While extending the life of the MiG-29 into the 2030s is theoretically possible, actually doing so will be real breakthrough in Russian aircraft capabilities. The way things are going now, the exported MiG-29 will still be flying, while those in the Russian Air Force will be grounded.
MiG combat aircraft have a long history of poor design and nasty flaws that limited their usefulness. But the basic stats of MiGs have always been good, and the idea was to build and use lots of them all at once to overwhelm the enemy. MiGs were not designed with experienced pilots in mind, but rather for a guy who can carry out some basic maneuvers, and do what he's told by his controller. The makers of the Su-27 realized that air combat had changed, and quality now trumped quantity. MiG tried to make that transition with the MiG-29, but was unable to shed its old habits.