Here is another way to look at it. PAC has no prior design experience of making a fifth gen aircraft. It is foolishness to expect a jump straight to fifth gen. On the other hand, Thunder is the perfect platform to test various fifth gen concepts. No one is talking about a light weight fifth gen aircraft. But there is a definite possibility of seeing fifth gen concepts on Thunder. As an example, inlet design may be so subtly updated that your eyes won't be able to tell the difference, but on a heat map it can lead to drastically reducing the heat signature. It doesn't make Thunder a fifth gen aircraft, but it is a fifth gen concept. Similarly, key areas of the aircraft can be optimized to reduce the radar signature. This includes a stealth nose and inlets, judicious use of composites and RAM coating, managing the heat signature, controlling EM emissions etc. And then there are the software features such as LPI AESA radar, AI, HMD/S etc. If a higher rated engine is used, you can get more power for these electronic features. Another 4+ gen feature is super cruise which can be possible by a combination of better engine and materials.
The point is, there are many avenues left to take Thunder to the next level. I do not believe the design has been maxed out yet.