You are misinterpreting the situation! Yes there is a lack of offensive capabilities and yes there is a need to augment certain roles, but not with additional MKIs, but IN ADDITION TO MKI!
The upgrade of the Mig 29 and Mirage 2000 are offering exactly what you want, they add offensive capabilities to IAF, in different roles and in addition to the MKI and it's capability. The whole talk about Brahmos addition is hyped and makes people to overlook, that there are hardly other important weapons added to the MKI. The plan as far as we know is, to add Astra and Sudharshan to replace Russian weapons, but nothing crucial that would make it more capable in new roles. On the other side, the Mirage 2000s gets SPICE 2000, heavy stand off weaponary (I am disappointed though that Scalp wasn't added yet). Why are there no SPICE 2000 or even better SPICE 1000s to replace Russian bombs, while giving far more stand off strike capability?
And even worse is the comparison to the Mig 29UPG, that "was" only useful for air superiority as you stated, but now will augment the MKIs in SEAD with Kh 31, in CAS with Kh 29 and KAB 500 and even will complement the MKI in maritime attack roles with the Kh 35 (that the MKI doesn't have yet) and that offers not only a far more cost-effective, but also more versatile anti ship option, since not every naval target requires a Brahmos or Brahmos M (see the discussion on anti ship missiles here:
Indian Navy News & Discussions | Page 199
so the Mig gets far more offensive capabilities than the MKI with the upgrade and will add far more roles to IAF's tactics, in addtion to the limitations they had till now with basically only the MKI as a multi role fighter.
Rafale in this case is a whole different issue, since it's on the one side a medium class fighter with similar roles as the Migs, quick reaction interception, CAS, anti ship..., but because of it's high capability, it adds even in roles deep strike or air superiority roles, that the MKI currently occupies (mini AWACS = long range detection capability, that can be diverted to other IAF assets, high maneuverability, AAM loads and endurance, heavy and cruise missile strike capability...).
So while IAF is increasing it's offensive capabilities besides the MKI, by crucial upgrades of the Mig 29s and M2Ks, the Rafale will once again push the capability to another level and actually gives IAF an equally capable alternative to the MKI too, although that wasn't the intention of the MMRCA and that is what IAF will love the Rafale for. They won't be dependent on the MKI alone, not on Russian weapons alone, will be far more flexible when it comes to tactics, offensive and defensive capabilities and that at most likely less operational costs and maintenance problems (lets ignore the industrial side of the competition for a moment).
Additional MKIs would only mean a higher number of the same fighter and the same capabilties, but no benefits in tactics..., while remaining with the same downsides too (costs, reliability, dependence on a single type of fighter). And to make you understand that this is not only a pro Rafale statement, I am still for 40 x of early Pak Fa from Russia (although IAF rejected them), instead of the 40 (+2) x additional MKIs from HAL, because they actually would add more offensive capabilities to IAF, would give them a new topline fighter next to the MKI, just like the Rafale would be, just with even more capabilities. So any fighter that makes IAF better, NEXT to the MKI and it's capabilities should be welcomed, while blindly adding MKIs for the sole reason that it is a good fighter doesn't make IAF better.