There's a thing called organizational death spiral. You don't have to down 100% of aircraft to cripple the Russian Air Force.

The less planes they have, the more they have to use the same aircraft, the more airframe stress that accumulates on those airframes, the fewer aircraft they have to rotate in during repairs and maintenance.

https://www.forbes.com/sites/davidaxe/2024/02/29/shooting-down-11-russian-jets-in-11-days-ukraine-nudges-the-russian-air-force-closer-to-an-organizational-death-spiral/

Also:

https://breakingdefense.com/2023/10/russias-air-force-eating-into-aircraft-lifespans-with-no-easy-solution/

Modern Russian aircraft are designed for a 3,500 and 4,500 flight hour service life, and some for as many as 6,000. But the Soviet-era platforms were designed for 2,000 to 3,500 hours. Some models, such as the MiG-31, have been upgraded to extend service life, but most of the older models are nearing the end of their service lives with only 500 to 1,000 hours remaining.