I'm going to repost my thoughts from the other thread because everything got downvoted without anyone really addressing the crux of the matter.
So what happened?
Russia lost 4 combat aircraft due to mechanical failures over the past month - source. Can't recall planes going down at this rate due to mechanical failures.
To compare the figure the rest of the world lost: 11 fixed winged aircraft in 2015. Four of which were American-made. 2 Soviet/Russian. Source - the list is incomplete but whether it's missing just the latest losses of the Russian Air Force or the numbers are inaccurate for the rest of the world - I'm not sure. If someone could find a better source, it'd be interesting to compare.
What is the significance of this?
The point is you can have a modern plane but if the maintenance is not there its combat effectiveness is reduced dramatically.
For example, the F-16 requires 19 hours of maintenance per flight hour, the F-22 requires around 30. Source[1] - in the source it's actually 45 hours for the F-22, but they cut it down.
There's been a lot of talk recently about pure numbers of aircraft (and the navy to some extent), ie we need 50 or 100 of PAK FAs. But I think the question is whether the Russian Air Force can support these numbers, while purchasing Su-34s, Su-35s (as well as, Ka-50s and Mil-28s) because maintenance is a very different animal.
The Ukrainian army is a perfect example to illustrate this idea - what's the point of having thousands of tanks of none oif them work properly? Not saying that the state is quite comparable but if you want to run a fleet of PAK FA - you will need an army of highly trained engineers, electricians, software professionals etc. Think of the T-34, it was good/popular because it was simple, same goes for the AK. But this approach just doesn't cut it these days.