I think there are still substantial issues withthe system - it should always be used with all the tech as you say, but they also need to make sure it is a consistent process, because to my mind England have been on the end of a few this winter that have shown up flaws in the system.
Having watched a lot of test cricket this winter, I think there are still a lot of issues with the process. When India refuse to use it and you have test cricket in New Zealand which has all the options available to them, it can't be consistent process without all technology being available to the tv umpire.
In the case of Cook and any other referral for an edge, unless there is significant evidence or an obvious deviation then the tv umpire is guessing. The 3rd umpire is there to access the overall picture and to eradicate any doubt, they surely can't do this in this series.