The dip was pretty minimal. Let's be honest, the Tonga player smashed the Scotland player in the head with his shoulder. That's always been considered highly illegal and dangerous.
I don't agree about the dip being minimal, and clearly neither did the bunker ref.
To summarise my thoughts;
The tackler could have gone lower which would have resulted in a totally legal tackle.
The tackler made contact with the head so the tackle was dangerous and illegal, but there was mitigation hence yellow rather than red.
If there was no mitigation then it would have been red.
That is under the current rules that the refs have to follow.
The legal tackle height will need to be lowered or else the game has no future (will be sued out of existence).
Whether a lower legal tackle height will destroy the game is yet to be seen.