Ehh the NGAD/F/A-XX program isn't going to outright replace the SH, at least not immediately. The USN is still acquiring new SH platforms, the latest of which have conformal fuel tanks and an enclosed weapons pod. They have also funded refit and acquisitions of the SH through the mid 2030s. I suspect the platform won't be abandoned til a lower cost option becomes available (maybe a carrier based UCAV). It will be supported for another 2 decades at the very least (by which point India should have competing local designs and even indigenous 5/6th generation platforms).
The NGAD will start replacing the SH from day 1. Production of the navy's last SH is set to end next year. Naturally the completion of the replacement itself will take more than a decade, until 2050. So even the last SH inducted in 2021 will see service until 2050.
I agree the SH isn't suitable for high altitude warfare. The strengthened undercarriage is an unnecessary weight penalty for the air force. I do still think that if the IN is going to import a carrier based fighter, they should go with the SH. It meets their capability requirements, has a well developed supply chain, fits on the elevators and shares engine commonality with all the proposed indigenous fighters. It also helps the IN integrate with western forces, namely Australia and the USN, who also operate SH and who will be the primary strategic partners against an ever expanding PLAN.
It can't fight the PLAN though.
Although, all of this is a moot point if the IN is serious about the TEDBF. Why bother with a 4th gen import (however advanced) if you plan on adopting a local version in the near future? If the IN really does want to tie in with the IAF, then they must be doubtful on its performance or the delivery timeline. Or they have massive irreconcilable issues with the Mig-29k and want to replace it asap.
The problem is the carriers itself. Due to the limitation of the carriers, the IN is forced to make an aircraft that's actually suitable for the carriers. The main reason for MRCBF is the third carrier. They need a CATOBAR jet operational alongside the production of the third carrier. They planned to operate some of the MRCBF jets on the first two carriers as a complement to the Mig-29, whereas TEDBF is meant to completely replace the Mig-29. So both meet two different requirements. Especially considering TEDBF is expected to be STOBAR capable.
As for why they want 4th gen for both, it's proven and the navy has to rebuild its CATOBAR experience from scratch, which is going to be very difficult with an immature 5th gen design.
The biggest issue is that naval Rafale is a different variant from the standard model, whereas the SH has no other variants and will gain cost benefits from more streamlined production. I do agree with the logic of just sticking to one platform. The SH should have been chosen when the initial mother of all deals competition happened. Now that the IAF has acquired from Rafales, they should just stick to it for their medium weight demands instead of complicating their logistics with other imported platforms. Currently India has chosen to flounder and not acquire any platform in significant numbers until their adversaries catch up and surpass them though. A bold strategy, will be interesting to see how it works out.
The SH was fine back in the early 2000s, but the minute 2010 came in, it became outdated, and today it's almost obsolete for some roles. The only thing relevant on it is the AESA radar, but now pretty much all new jets the IAF will induct from here on will have AESA, starting with the Jaguar, so the SH has now become a contemporary jet to the IAF, not even an advanced jet. The Advanced SH had some chance of being relevant, but the Block III is merely old wine in a new bottle. Regardless even the Advanced SH would be pointless since it has reached the end of its development cycle and has no MLU program planned.
Rafale today is like how the SH was back in the early 2000s and still has a good 20-30 years before it becomes obsolete, which is around the time the French will start replacing the Rafale as well.