Total Russian losses of multirole/fighter/attack aircraft are estimated at only 42[1] compared to a possible in-service count of approximately 1228[2], for a 3% loss so far.

Lack of industrial capacity to produce aircraft parts and munitions is a more likely reason for keeping Russian aircraft on the ground and slowing the pace of the war. It is estimated that Russia in the first three months used up an equivalent of 4 years worth of US production of all types of missiles.[3] And the US has so far sent Ukraine approximately 4 years worth of production of Javelin missiles.[3] The US aren't expecting to be able to double the annual 2100/yr production rate[3] until a few more years time so the limiting factor for Ukraine will soon be industrial capacity of Western countries to manufacture new munitions and equipment.

Impacting all parties to the war is global inflation that is significantly reducing defense industrial capacity output in real terms. For the US, the defense sector contracted 6% in real terms in 2021.[4] With Russia's official inflation rate recently over 17% and many sanctions in place, defense sector output for Russia is a far bleaker situation.

[1] https://en.wikipedia.org/wiki/List_of_aircraft_losses_during...

[2] https://en.wikipedia.org/wiki/List_of_active_Russian_militar...

[3] https://www.rusi.org/explore-our-research/publications/comme...

[4] https://www.iiss.org/blogs/military-balance/2022/02/global-d...