To the best of my knowledge, there are no funding problems with S7.
Chances are that such is the size and importance of their domestic network that if they did run into problems, the govt would intervene.
Can't speak for jet fuel prices in Russia, - fuel at the pump is about $1 a litre - but I've done a few domestic flights recently and their fares, on a cost per mile basis, are similar to those in western Europe. All the flights I travelled on were full, and I know it's not always an indicator, but the Russian domestic carriers would appear to be doing reasonably well.
(Long-distance train fares have gone up significantly in recent years, but it's still difficult to get seats on them at short notice, which may explain why the flights are busy).