Okay...
As discussed over a 1000 times in the past, what options do you have?
-Rafales
-Gripen NG
-EuroFighter
-and the Ruski trash.
New type (used) means we gotta start from scratch. Training (Pilots & Engineering), Spares, Weapons etc.
The reference you've given for the S-400, well it appears that even the mighty F-22 (Raptor) & the poor man's Raptor, the F-35 don't stand a chance against it. So that debate should be set aside.
As for the Rafale, it'll take India years to master it. They still haven't mastered the Su-30MKi - so good luck to 'em.
We on the other hand have mastered the F-16 for decades & upgraded versions of 'em are plenty for the next couple of decades.