why do they not just talk to each other, rahter than trying to make the decision in isolation - in the same way that THlar was trying to guide Alibert in France.

Rather than the split second decision by the on field ref taking primacy, they have a sensible conversation about what to look at, then review it together in real time - with the on field ref watching on the screen.

Let Barrie McDermott decide - he likes to award the try based on effort and desire; seems reasonable.

The problem inherently wrong with the system is that it doesn't take account of multiple factors.
For example, in a game recently a referee gave a "no try" decision based on a player putting his foot on the touchline before grounding the ball. The video reply clearly showed this was not the case, however there was some doubt that the ball wasn't grounded properly. The video referee (rightly according to the guidelines) stuck with the "no try" decision based on this, but this was not the reason the on-field referee gave the initial "no try" decision (indeed, as he didn't ask to check the grounding it is reasonable to assume the referee would have given a try).
Also take the Solomona "no try" in the Cas v Wigan game. The referee gave a "no try" decision based on a double-movement. Even though there was no double-movement, the VR stuck with "no try" as there wasn't a clear shot of the ball touching the line. However, this wasn't what the on-field referee asked the VR to look for, so again it can be assumed he was happy that the ball touched the line and if he hadn't suspected a double-movement, the try would have been given.

The other issue, of course, it the quality of the VRs. Whilst ever we have blatant errors (tracking the wrong player, not looking at the correct incident etc) then the principle is flawed.
I'd remove it completely. However, the key would be to also remove the big screen from the ground so that referees don't get immediately berated if they make a questionable decision.
For me, the VR has sanitized the sport and taken away a key element of the game. When my team score and the VR is in operation, it's hard to celebrate a try at the point of scoring as I know it's likely to be two or three minutes before the decision is made. Likewise, when the opposition scores, I'm hoping the VR can get us off by finding a technicality and disallowing the try.
I much prefer the decision to be made immediately by the officials on the field and I can accept they might make genuine mistakes. But it's embarrassing when the bloke with all the camera angles available does the same.