What I don't understand is the rainjacket 'management' and choice:
About choice:
I still see some flappy designs or designs that seem to be no stretch and some jackets with a shiny fabric. I would expect that the flappy (catches wind) / non-stretch (hinders racing) / shiny (not very breathable) jackets are a no-go. What works when it's pooring rain is either a small rainjacket without sleeves (I saw Ineos ride in those) and I know Gabba is a pretty good design (non-flappy, stretch, breathable and still relatively rain-resistant) and I would expect most teams ride with that or something with similar function.
That's for the choice, and I honestly don't know if all teams do that well.
The other thing is the rainjacket management:
When to wear a jacket and when not, is imho mostly determined by someone in the car having a good app predicting chance for more rainshowers, and a look around to see if competitors are still wearing or not.
Next, there is something to be said about how to put it on / take it off. If it's on at the start and off halfway the race, it should be fine to do it on the bike. But if it has to be on in the race, a non-sleeve jacket will generally work, but most jackets with sleeves are very hard to put on while racing. You could as well plan to put one one by having a controlled stop (like in a bike change) with some team mates. That would probably save more energy than to risk it to do it on the bike, or not dare to put one one and get cold.
Last, there is something to be said about changing a wet jacket for a dry one. I hardly ever see someone doing this, but I would assume it could be beneficial. I thought Nys did this in the race?
What's for sure is that Evenepoel not only made the (rookie) mistake to take it off too soon, but there was no plan at all what to do when it would start raining again. The team could either decide to ease up and allow Evenepoel to put on a new jacket, or they could have provided him or some of his helpers with small sleeveless jackets that fit in the back pockets, or they could at least have warned him through the radio that it was a big gamble to take it off with 75K to go.