India aren't getting Russian equipment, they abandoned the PAK FGFA, and going to an indigenous AMCA option. There is no signs US has offered F-35's to India given the CAATSA violation with Indian import of S-400s which can detect F-35 signatures (Same situation with Turkey). Indian refusal to condemn Russian invasion only further divides the QUAD. They aren't even seeking off the shelf Su-57Es.
So the reality is India isn't forced to choose between Russian equipment (canceled joint FGFA project) or American equipment (isn't even offered F-35s due to CAATSA violation with S-400), but has a third option which is indigenous innovation. So don't push some false dichotomy arguments to justify Russia selling top arms to China's enemies while being in US-UK 'special relationship' style alliance.
I was speaking in more general terms, but if you are so hung up on the fifth gen fighters, you need to consider the following,
1. Judging from the fate of the Su57, the Su75 will be delayed for quite sometime. By the time it comes out, if it comes out, a sixth gen fighter maybe close to being ready for China and the U.S. At that time, especially given how slow the Russians were able to produce these, it may no longer pose a threat to China.
2. If J-35 is ready long before Su75, it will take most of the non-Western market such as Iran and Turkey, which would put the viability of the Su75 in question.
3. At that time, especially if Pakistan will be equipped with J-35, India buying a fifth gen plane might be a blessing in disguise for China since more sales to Pakistan and maybe other countries connected to Pakistan.
All military equipment becomes obsolete over time. It may not matter if it comes very late.