MKI upgrade would still be much more limited than Rafale certainly. I was more emphasising the air superiority BVR role where RCS matters a little less. With AESA upgrade and R-37M, a "Super MKI" Would be pretty much equal in BVR air superiority as Rafale. Where Rafale would be stealthier than MKI, the MKI carries more and flies further/longer.
Su-30MKI, like almost any flanker sans E and likely J-15B, is a roadside lightpole in terms of RCS. Even with updated jammer(ISPJ) it won't be quite there, those are ultimately related metrics.
Unless 177-ied (which is not a commercial proposal off the shelf just yet), is underpowered. Significantly.
Su-30MKI doesn't quite match Rafale in strike options array, and overall is directed in a different direction(stand off "javelin thrower").
Finally, like most flankers, Su-30MKI doesn't like low altitude flight all that much.
Rafale is far more capable than upgraded MKI but they really don't need much more anti-surface advantage over Pakistan so the ability to strike effectively is pretty much a given. It's India's inability to take air superiority that is a greater issue for IAF and withing the scope of fighter upgrades/acquisition.
Is it though? Indian prime strike right now is Jaguars and Mirages(not exactly in their prime even with LLU upgrades). Indian medium weight fighter is mig-29upg, which is f-16 blk50 equal and nothing more than that - and their remaining life after MLU is also not that long.
This part of the fleet needs replacement.
As for air superiority - ultimately, all upgrades and new procurement contribute to that...
Yes, higher level view, those two platforms are absolutely complementary and MKI upgrade is not a substitute or alternative to Rafale. It's just that it should be a concurrent if not a prioritised action for IAF rather than fresh Rafale acquisition.
They tried "make in India" instead of paying Sukhoi, tried(and still try) really hard. Good intentions paving road to hell.
Tbf, Sukhoi doesn't have an immediate good "off the shelf" upgrade for Su-30MKI either; Su-30SM2 is a rather cheap upgrade of a very secondary(as for Russia) aircraft).
For India, on the other hand, it's their all - heavy fighter with largest sensor, strategic deterrence(brahmos), stand off strike(rampage), escort EW(SAP-14). They need a lot.
Rafale with its puny radar could never detect PAF fighters at the ranges it got shot at, let alone track and engage them.
Rafale RBE2 has more or less similar detection range as Bars, and it's fully sufficient v respectable 3m RCS target.
Let's limit this "puny", it isn't a mig-21 radar after all. It's AESA, liquid cooled and larger than one on JF-17. Whatever problem Indians had in may, it isn't radar.