Long post incoming, lots of guesses and unproven speculation
and probably a bit too much about Mercedes for an RB thread, but here goes:
I don't think there is any one thing that started to suddenly work on the Mercedes car, IMO that is not their approach, I think they had plenty in hand at the beginning of the season, and RB had slightly less - meaning, both of them were running at lets say 90% the first few races, and race by race they use up their performance reserve and gradually increase the cars speed to see how the competition react, and about the time Marko said - we're in trouble, what it meant - RB had used up all their saved tricks and there was nothing left in the pipeline, while Mercedes continued to become quicker and quicker, had Mercedes showed their full hand at the beginning, RB may have thrown in the towel and just put all the focus on the new car, which would put Mercedes in a compromised position for next year, and RB also could update their models for various bits of the Mercedes car, which is also undesirable, and I'm 100% sure they are building models not just for their own car, but for competition as well, it just makes all the sense to me.
Mercedes season long strategy has been far superior to any other teams, that of course has a lot to do with their engine development, which allowed them to explore other options more freely than the other teams, other teams thought - this one thing will give us 0.3sec and we'll be back in the hunt, and when they do match Mercedes, Mercedes simply use the next thing they had lined up and they are again untouchable in the next race - that is crushing for competition, and it happened season after season after season, because the ones trying to catch up essentially don't know the true deficit and thus cannot make the necessary decision to either continue their current design philosophy or switch to something radically different, because the current concept will clearly be exhausted and won't come close to Mercedes performance. I think this has been happening both on the engine front and the car, and because Merc knows both of these - they can then coordinate both branches of development to maximize the overall performance - that is one thing that customer teams absolutely cannot do, and this effect was noticeable in Mclaren-Honda years, when the car development expected a more powerful engine, but when it was clear that there will be less power available, they had reduce the df/drag on the car to not be sitting ducks on the track, which then leads to bad tire performance and overall the car just doesn't work, similar effect was noticeable when Ferrari were forced to stop using the "trick" on their engines, suddenly they were no more than a backmarker.
And because teams are closely involved in the token system, they can then use this very system, that was put in place to even the competition, to essentially prevent the competition ever reaching their level of performance, and I bet Mercedes took all advantage they could out of this, they have been sandbagging like nobody else to show they are not that far ahead, and thus not turn the token system into a joke, and during the season take one bag out at a time to imitate "normal development" - I think this was also why someone in the Abu Dhabi did what they did.
I'm not going to debate any of this with anyone, just thought I'd put my thoughts down, I'm not saying Mercedes cheated or any of the sort, just that they play a bigger game than the other teams, on more fronts, but this year RB had tucked away enough cards to mount a reasonable challenge and even that would have not been enough, I'd really hate to be competing with Mercedes in the long run, they are just on another level in that respect, hence the 8 straight titles, and I don't the cost cap thing is ever going to achieve anything to fight such dominance, it is just so easy to circumvent and make mockery of, and if Merc agreed to it, I'm sure they have already stacked their deck for that fight.