The reason IAF Retired the MIG-25 is because the Foxbat's only role in the IAF was Reconnaissance and Surveillance. Later When Satellites came to be and could do these roles, Better, cheaper as Compared to the MIG-25.
So hence IAF decided to retire it. Also the MIG-25 Required lots of Maintenance and was expensive too. So Sukhoi SU-30 MKI has nothing to do with MiG-25 getting replaced as far as I am concerned.
Well Mig 31 is essentially an improvement overthe Mig 25 so as you have requoted satellite made the reconnaissance part useless.
Other advantage was speed where Mig 31 only excels to intercept big radar etc.
Well Sukhoi provided more... Big radar can intercept with long range missiles and unlike mig 25/31 can dog fight and provide air superiority.
So Sukhoi essentially ended the need of Mig25/31 other advantage too.
Regarding maintenance if we really needed it we would have maintained it somehow or bought mig 31. But with Sukhoi and satellites etc there wasn't much need.
For Russia having such a very large territory it might be meaningful to still have them.