In that case can you tell us what the role of the MCA is such that the PakFa, LCA, and MMRCA cannot accomplish operationally? Why is a middle plane needed? If we look at the USAF, they only have a light/heavy combination for fighters.
Its ironic that in the midst of a conversation which focuses on how Indians ape the US, the above statement pops up, asking why we should not do what the US does! India does not believe in a light/heavy combination of fighters. That is because it needs a mix of capabilities which the above combo does not provide. The US will have (even after retirement of the earlier ones) several hundred F-15s along with 187 F-22s. They are going to be backed up by over a thousand F-16s iteratively replaced by F-35s. To support this massive fleet (and check the weight of the F-35, its no light fighter but has the highest thrust engine ever developed for its weight) the US fields a plethora of AWACs, ESM support, refuellers etc. Plus it has bombers!
In India with a 39.5 squadron cap, we have opted for a mix of light-medium-heavy for our tactical requirements. The light LCAs will mostly meet Pak, whereas the medium-heavy fleet can be used against deeper PRC assets. The medium offers a lot of the capability of the heavy fleet with lesser operational costs. The IAF's budget is not infinite.
Given that, the replacement for medium weight fighters needs to be something similar in terms of weight (and operational costs) but more capable. The MCA is intended to be that system.
As to why the PAKFA and MMRCA/LCA cannot hold their own in the future, they can, but you will have to pay through the nose to surround them with support assets ..the PRC is setting up one of the world's most expensive and capable SAM networks with layers of defences. Long range PGMs/CMs are expensive. Relying on conventional 4-4.5 G aircraft, armed with limited range munitions against a network which will encompass even tactical air support is going to be deadly. Like it or not, RCS reduced platforms will be required.
Unmanned systems can assist but they cannot replace the capabilities of manned systems given current and projected limitations in bandwith, realtime processing and decision making and complex decision making.
It is the same reason why the US is investing so much time and effort in the JSF despite having the option of iteratively developing the F-15, F-16 and even the F/A-18 E/F which last aircraft, is essentially a new build aircraft.
The era of manned cheap "Gnats" is effectively over in the Indian subcontinent given the kind of systems that are in place and are being put in place. The Israelis, are wary of the S-300 system, the IAF faces batteries of the S-300 PMU2 already.
These projects are tied to the MCA because resources devoted to developing the MCA can be used more efficiently elsewhere, to daydream a little bit - More variants of Tejas - Tejas stealth version, Tejas UCAV, Tejas EW, ENGINE etc. We could also develop anti-sat weapons that can be mounted on fighter planes (US invention!)
Again, mistaken! Resources tied to the MCA will not get used up in entirety for future products of limited scale you envisage. If these are not used, these capabilities built up with so much effort via the LCA will wither away and only laments will be heard later, with ample finger pointing on who exactly was to blame!
Furthermore, the IAF has never asked for "LCA stealth" or "LCA EW" - by the way, how exactly can the LCA have the power generation capability and loiter of a dedicated EW platform! I would rather look at something like the AN-32 or IL-76 or MKI for a dedicated jamming role.
The MCA is driven by IAF requirements of a medium weight aircraft which is sophisticated enough to meet credible threats and is reliant on local technology, ergo local product support and upgrade ability. The alternative is to import and kiss goodbye to operational sovereignty unless you make significant concessions elsewhere. India is no Israel where topup munitions stocks and airframes will be flown in during the middle of conflict, with say Pak or PRC
The MCA is the culmination of India's efforts to develop a worldclass aviation industry, I find it bizarre that claims are made that it be shut down for the benefit of a PAKFA - the same manner in which "aluminium models" of HF-24 Maruts follow ons were derided while we went for the "sensible" option of licensing Jaguars and MiG-27s, only to be caught with the realization 2 decades late, that we had been subsidizing other folks aircraft industry's and effectively cooked our own goose.