Not only that it is/was a tech demonstrator but also being just a prototype and that too the first prototype it was never meant to be pushed to the limit and is always flown with great care and cautions! The second prototype will be a marked improvement and it will shows its true colors only once SBP units are out flying. On paper, it looks a tailor made solution to our future requirements.
Regarding the WHY part of the question, well because 5th gen is the future of air combat and because the big players are already working up 5th gen+ options so it is time we roll up our sleeves and enter the era. Also because joining a program at early stage will mean that we have all the required know how of the plane when it is inducted in numbers and also it will give us ample time to master the plane and get the max out of it. This is something VERY important and something that is quite often overlooked on online forums. You need YEARS to master a plane and get the best out of it. This is part of the reason why we still love the F16s!! We KNOW THEM!
One other important reason, that too missed sometimes is our JF17 project. That is still developing and the plane fits the bill and meet our needs perfectly, especially the $$$ part of it. So joining a 5th gen project now will mean LOADS of goodies integrated into the JF17 future batches as well and pave way for making JF17 relevant for decades to come.