Regs were changed to comply with Breretons view on how an airport should function.. Howard had his input too.

Agreed. In the late '90s we were having to do stupid stuff like land on 07/25 with 20 knots crosswind and 5 knots tailwind. Or similar with 16L. Sometimes we even had to hold for the privilege of landing on the single, short, runway with tailwind, simply because the time of day specified which runway Config should be used.

AIPA put out a directive to not accept more than 15 knots crosswind when it was being applied for noise sharing. Note the important distinction there? We'd happily accept 20knots if it kept the flow moving. We wouldn't accept it simply because the time of day specified we had to when a more efficient into wind configuration was available.

I have no expert or inside knowlege, but I do know that MEL applies exactly the same XW/TW wet/dry as SYD, so I seriously doubt it is politically driven or airline driven. Probably closer to the truth would be ICAO standards.

Yes, AIPA did issue such a directive many years ago, but that was a response to political noise sharing (exactly as Keg said). It never had any influence on SYD airport modes to my knowledge.

Yes it did influence the mode operated as the requirement to land 16R often led to a change back to parallels ops.

The XW criteria was 25 knots, its now 20 knots. As I said it could be 15 knots or could be 30 knots. Its up to CASA and I guess the airlines. Increasing the XW criteria "too high" could lead to more off-mode requirements which have safety and efficiency considerations. Examples - landing aircraft requires RWY 25 when operating 34 parallels.

So what happens somewhere like Brisbane when the crosswind is 21+kts? Close the airport or just deal with it as per the training and aircraft capabilities?

What happens around the rest of the world?

When the crosswind is 21+kts?
Quote the wind and get on with things, however when there is an alternative runway with 20kts or less then quote that runway.