It's flight controls. We created an unstable object and gave it the power of flight. This is easily the hardest part in designing a fighter aircraft and we had to do it from scratch. This is the only reason why it took so long to develop the LCA. It took us 25+ years to get it right. We also took a long time to get the experience necessary to certify it.
Now that we have managed to successfully do it, all future programs will be designed and inducted much sooner than it took to induct the LCA.
From a broader perspective, we had to invent a lot of stuff, or re-invent if you must. We had to get the airframe design right, then we got the flight controls. We had to develop all the sub-systems from scratch, like radar, EW suite etc. Invention is the hardest part and can take a long time to accomplish.
From now on, for all successive programs, we need more innovation and less invention. Innovation is much easier to accomplish than invention. It's about making better versions of existing designs. For example, it took more than a decade for us to design and induct a mechanical scan radar. But Uttam was designed and developed in a shorter time by the same lab. The design is going to continue to evolve.
It's the same thing with the engine. We need to get the basic building blocks ready, which could take as much as 30 years, once done, we can start designing and producing prototypes of new and better engine designs in just a few years.
LCA Mk1's FOC will signal the maturity of our core technologies that's necessary to develop the AMCA. With the flight controls fully operational and certified, the development of AMCA will happen really fast. After that's done, adding avionics will be subject to what's available. We don't need the French for that.