Firstly if we want to go for Fifth Gen, we anytime can buy Su-57. No one stopping anyone to do it. and no it is not a technical constrain. It is actually budget constraints.
The Su-57 fits into a different niche. It's not a replacement for AMCA or vice versa. And whether we go for it or not is obviously not a technical decision, but a budget and political decision. Naturally, you should be comparing the SU-57 with the American decision to go for F-15, the TEDBF can't be compared here since it's purely a technical decision.
And in a situation when the US budget is declining, saying they start to go like a cold war purchase is not only stupid but idiotic too. Budget constraints are a big thing, even for the USA. The US has now 1200 F-16 right now, and around 500 are already retired. The "PLAN" of 1700 F-35 replacement is totally on par with replacing F-16 on one on one basis. They are not even touching replacing F-15 with F-35. And again this is only a plan, this plan is actually 50 years long program in procuring 1700 F-35. It is not like in 2022 US will have 1700 F-35? So what are they going to do until 2070 if the 4th gen becomes obsolete tech as you suggesting?
Their budget decision has less to do with having no money and more to do with priotities of spending. If their security situation gets worse, then their budget priorities can be reversed at the drop of a hat.
The original replacement of the F-15 was the F-22. Now their only choice to replace the F-15 is either the F-35 or another F-15. As far as the USAF is concerned, they would rather replace the old F-15s with F-35s rather than the F-15EX, but they have not been allocated enough money for that, hence the F-15EX it is. Also, the F-15 decision is still pending, right now they have only committed to 12 aircraft and 2 squadrons.
What's more interesting is the USN's move. They had earlier planned to induct 72 SH B3s, but have since decided to cut those numbers back to 36 and invest the remaining money into the development of the NGAD. So they are only going to receive 12 SHs each year over the next 3 years.
Anyway, you are wondering about 2070, but that's only old aircraft with long lives bleeding into next gen territory. It's not any different from the IAF still operating 3rd gen aircraft like the Jaguar even today. So any 4th gen aircraft being inducted today will see service for the next 40 years at the minimum, ie, 2060. But none of these aircraft will be the primary aircraft, they will all only end up supporting the primary aircraft. No serious air force is buying or planning to buy newly designed 4th gen jets in the 2030s. Only the IN is, and out of desperation than anything else, and for them a 4th gen jet will be their primary aircraft in the 2030s.
Now to move forward, to India. If you are suggesting we have a very deep pocket, then why not purchased 123 Rafales by now? If it is not a budget decision, and we are Saudi Arabia, we should buy Su-57 or KFX. Why not? Because firstly AMCA is a strategic project, with requirement even in SFC (and that is totally different ver btw), and we are actually suffering from Dollar/Euro constrain. And ORCA is not like a brand new tech going to develop, 90% of LRUs, engines, avionics going to be as same as in Tejas Mk.2. The technological maturity is already there, the only thing you need is airframe maturity, and it can be achieved if HAL dedicates one prototype airframe for stress testing only.
You are confusing one issue with the other. Budget issue for buying numerous jets is different from a budget issue of developing a fighter jet, both are under different overheads and is not related to what I am referring to.
So, no it won't take 10 years in maturing a platform if designed smartly (the same way Koreans doing in KFX), and I pretty sure we adopting the same path.
It takes a minimum of 5-10 years before a jet becomes operationally viable. This is done through numerous large formation exercises. This process begins after a jet achieves FOC. It takes 2 years after FOC just to train your first batch of fully qualified pilots. Then it takes 2 or 3 more years before it starts achieving dominance in its field, that's when you get pilots with 500-1000 hours of flight experience. And after 5-10 years you will have enough aircraft and enough pilots for a high rank officer to qualify your fighter jet for role replacement of the previous aircraft. You can't magically churn out pilots overnight no matter how smartly you have designed your fighter jet because this has nothing to do with training a pilot. Hell it takes years just to write the instruction manual for it and then it goes through a long process of corrections, which also takes years. There is a reason why test pilots are so highly respected. IIRC, the MKI took 7 years after induction.
So this process is easier and much shorter on a proven jet than on a brand new jet because someone else, namely the French when it comes to the Rafale, has already finished all the necessary tests needed before its qualification as a combat-ready aircraft.
The IAF is being forced to buy 4th gen jets of a particular class today because they were denied the choice for more than 30 years now. Even then they plan on buying only proven, or in the process of being proven, 4th gen designs like the Rafale and LCA and through the 2020s. If you are expecting them to buy a newly designed 4th gen jet in the 2030s in the light of the above facts, especially with AMCA coming online even earlier than ORCA, then you most definitely do not understand how air forces operate. It's impossible for the IAF to buy both the ORCA and AMCA at the same time and put both through a long baptism before using it when better alternatives are already available, including the Su-57.
The IAF will be replacing their Mig-29s and M2000s with LCA Mk2, Mig-27s with Rafale and Jaguars with AMCA. MKI replacement is planned for the late 2040s. With the exception of 32 MKIs supplied by Russia, most of the MKIs are very young and will undergo 3 overhauls pushing the fleet to between 2050 and beyond.