See, there is nothing called 6th generation plane as of now. Let me amke you understand generations here:
4th geenration planes are planes with FBW systems and some amount of automation. This came in 1980s. But as computer revolution happened in 1990s, there was development of better capabilities like AESA, inter-communication, EW, SPJ, sensor fusion etc and there came our 4.5 generation. With addition of stealth, it became 5th generation.
Since Indian IT revolution happened only after 2000, the Tejas which was conceived before that became obsolete even before it started. India didn't have any intention of making a cutting edge technology but merely wanted to replicate the 4th generation ones of other countries. So, some modifications had to take place. However, the biggest reason for delays was due to UPA government purposefully scuttling defence deals. This was true not only for Tejas but Also for other items like IAC-1, Scorpene, Nuclear submarines, Kaveri engine and many other defence modernisation, almost all of them conceived in Vajpayee time.
But, despite the delays, since the research had been sanctioned, DRDO had funds and they used it for developing as much as they could despite hostile government. The Tejas plane had limited sorties between 2009-2014 which also reduced the feedback and caused further delays. They also delayed Kaveri by not building a high altitude test bed which was crucial. The main grievance was in high altitude tests. The only way Kaveri could be fine tuned was by repeated testing. Flying it once to Russia is of little use at all.
Instead of understanding the real problem, you are pointing at arbitrary directions. India has no dearth of intelligent people. If DRDO is having too much of a red tape, then government can fund a private company via LIC and then hire scientists of higher quality to do joint development. There are several ways to bypass the bureaucracy problems if there is government will. But, congress was the thorn and has always been. Indian poos military research is due to congress scuttling them. India could have developed nuclear bombs in 1960s like China did. Nuclear submarines also could have been made in 1970s-80s and so on. Congress scuttled all of them under directions from their political masters abroad.