Maybe the Su-35 is just too good?
How Russia's Very Own Su-35 Might Mean Doom For the Su-57 Stealth Fighter
The Su-35S was intended as an interim solution; as a modernized air superiority fighter to sustain the Russian Aerospace Forces (VKS) through the 2010’s until the Su-57 fifth-generation stealth fighter enters serial production. But the Su-35S was almost too successful for its own good, ticking so many performance and role versatility boxes that it seems to cannibalize its more expensive Su-57 successor. In the short term, it’s likely true that there will be a minimal operational difference between the latest Su-35S units and the first serially-produced Su-57’s. As it currently stands, there is even a chance that the two fighters may use the same AL-41F1 engine.
“A unique machine, a deadly aerial fist,” is how the official television channel of the Russian Ministry of Defense introduced the Su-35S superiority fighter earlier this week.
TV Zvezda’s three-minute clip of a recent Su-35S training sortie over Syria provides close-up shots of the fighter jet being prepped for flight, taking off, cruising over the Syrian coast, and firing flares. On their youtube account, they published slightly extended footage of the same exercise.