You didn't think about the reasons why we currently need 2 platforms! The AN 32 can carry only around 40 paratroopers, an IL 76 over 120, so you will logically need 3 aircrafts for the same ammount of paratroopers. The difference between a C130J-30 and a C17 on the other hand is just 10 x paratroopers (92 vs 102). Even MTA will be able to carry around 80 x paratroopers too and the fact that C130s and MTAs will be deployed at more air bases around the country, makes them the logical choice for paradrops, since you have a suitable aircraft nearby and don't have to call the C17s and wait for till one is available (26/11!!!).
No, I agree that it has this capability, but that doesn't mean we would use this as part as our normal cargo operations. Tanks will be moved in higher numbers via road or rail transports. That's why we have dedicated wagons for Arjun, to transport via rail. The C17 can only carry a single Arjun and even if we occupy the whole C17 fleet, we would need a very long time to transport a useful number of tanks around, while no aircraft would be available for other missions. That's why this will be a very rare solution, in case road and rail links are destroyed for example and not a common policy.
The C17 is not a C5 or AN124, which are meant to carry higher numbers of MBTs. Even if we have to transport MBTs abroad for UN missions, we wouldn't use the C17s and would either lease an AN 124s, or ship them over => see recent Afghanistan, or Mali conflicts as well.
Again, the main purpose of C17s for India are long range strategic transports and high payload transports to high altitude areas. That are othe only roles where the C17 will have a decisive advantage over other aircrafts and for our requirements.
In any short to medium range, or medium payload missions, within or around India on the other side, the A400 class aircrafts are as useful, or even better.
Nope, the the real question would be, do you add 6 more strategtic transporters, or 12 x A400 multi role transport tankers for example, for the same costs?
The earlier would further improve the strategic transport roles, the latter the tactical, the strategic and the tanker roles and that in twice the numbers and no additional costs!!!
You have to see it like this, the C17 in IAF would be like the C5 in USAF (strategic transport), while the A400 in IAF would be like the C17 and the KC130J in USAF (tactical and strategic transports + tanker roles).
The US have to transport everything far away, be it tanks, or paratroopers, while Indian air force mainly is used in and around India, so range is not the important point, but efficient transports of payloads in different landscapes and climates, to dedicated air bases or unpaved airstrips and if possible with multi role capablities.
So the addition of A400 to the C17s, would offer:..
...more numbers (for the same costs)
...more variety of roles
...more available airstrips for operations around India
...more support capability (Tanker, special ops, paratrooper roles)
...more industrial benefits (because we still could take over parts of the production)...
While the addition of more C17, just offer more of the same capabilities in the same roles.