The flight plan indicates the required fuel load INCLUDING mandatory reserves to cover various eventualities like holding and diversion; and so the captain's decision to take flight plan + 15 minutes fuel is entirely legal and in keeping with both company and regulatory authority requirements in place at the time.
If those requirements are subsequently subject to review, I feel it is unreasonable to castigate the captain of this aircraft for shortcomings in the standard operating procedures that he was trained to follow.
As a secondary point, are we saying that if the Captain had more fuel, he would not have commenced the approach which led to the accident......? If he would have commenced the approach anyway regardless of fuel state, it possibly was a heaven-sent blessing given the subsequent turn of events that he didn't have more fuel on board. As well as fuel powering engines, it can also power nasty fireballs that kill people very quickly. Thankfully for all, this was avoided on this occasion.
Even with the benefit of hindsight, I don't think any of us can categorically say what would or would not have happened.
Continuing to argue on this one is not making any tangible contribution to safety as far as I can see.