Would've been a good replacement for the Su30 in the heavy category atleast. The payload capacity and range is nothing to sneeze at. Not to mention this thing flies with a lot of onboard sensors.
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Replacing the Su-30MKI with AMCA Mk2 would be more than good enough. The benefits it offers to India over a Su-57 design that India had 0% say in, are immeasurably greater. And anyway the oldest Su-30MKI doesn't need replacing till 2040, so why should the IAF be stuck in a program where it had so little say in, till then? A good Su-30MKI upgrade program will keep the fleet relevant till 2045 easily.
For the FGFA program, we would've had a Su-57 line at Nashik to replace the Su-30MKI line and have been paying royalty to Russia with forced imports of raw materials (as it was in the MKI contract). Nothing would've been learnt by our engineers and our own AMCA program would've ground to a halt since IAF cannot support 2 5th gen designs in large numbers. It was a JV only in name, since the IAF and HAL had almost no say in it's design or it's requirement definition phase.
With the AMCA, the IAF is involved from the get-go and has to take ownership of the program for it to have a successful 5th gen fighter program. They know it well and the alternatives are very limited to say the least.
As it stands, the RuAF itself isn't using the Su-57 to take on any Su-30SM missions. The bulk of the RuAF air defence effort is still in the hands of it's Su-35, Su-30SM and Su-27 fleet.