Let's try this again.
Strategy is determined by race distance, tyre allocation, and the tyre and fuel multipliers.
Vehicle choice is determined by the track characteristics, the car's specific strengths and, if strategy is involved, it's tyre and fuel consumption.
When you take one good look at the event, whether it be on Monday or a few days before via a tweet, you can know right away what the ideal choice of car is in terms of characteristics, and maybe what the right strategy is. The latter is more of a learning experience, but again, something you can't really start to know for sure, until you drive on Monday, whether you find out Monday when you turn your system on, or the week prior.
The only way I would say that knowing beforehand would materially affect the outcome is if droves of drivers were playtesting the next ruleset as soon as Nenkai drops his tweet. But that's not really your complaint, is it? It's probably not even happening at all.
Your complaint is that people will either preempt the optimal car and strategy and consequently take away diversity of the rosters for the first day of the new events (your words), or not bother to turn the game on at all because the surprise is ruined, and neither of these are directly caused by the tweets, which others have pointed out.
You don't have to change your mind, and no one can force you to, but ignoring people pointing out to you exactly why it's false that Nenkai is 'ruining' the Daily Races, and still saying that you haven't been given a reason to change your mind, I just can't wrap my head around.
To use your (granted, funny) book analogy, it's not like giving your son a book and telling him someone dies. It's more like getting the book a few days early, but still not being allowed to read it until the day it releases. You don't really get an advantage from reading the cover or the back, but if the book is rubbish now it will be rubbish on Monday.
'ruining the daily races' weren't my words and I don't put that on nenkai (in any case it's just hyperbole).
I have two data points. Daily races before dailies started being leaked/widely followed, and daily races now. And yes my opinion is that the meta develops more quickly now, which reduces the entertainment value of Monday races. The fact that people know the bop and game better is obviously a confounding factor. And yes I agree it is only a matter of hours before the meta forms even without the leaks, and the topic is not worthy of as many off topic posts as this has now become (people sure get worked up and feel the need to assert their opinion and wishful authority on what people should post here- without that we would be sitting here with just an unanswered question to nenkai). But like I said in my original post, those hours were always the best of the week for the game. So yes I think it's possible that leaking the races has an adverse effect. And I don't see any potential benefits of leaking them, so why not ask the guy to stop?
But to address your counter points. Firstly, the streamer and A/A+ ranked driver community where this has the biggest effect is small, so the meta spreads quickly cause everyone wants to compete. Kie usually streams daily race testing every Sunday as an example. Right now I looked at open lobbies and there are two gr1 le man's bop lobbies with a decent amount of players (one is even labeled next week's race in Spanish) and it's only Thursday. Can't give anything but anecdotal evidence that people test the combo in single player on the weekend. But you can see how even in this thread how a supporter of the leaks says that's exactly a benefit of them. Can also check the daily race thread, and various streams.
The fundamental problem with daily races is there is always one strategy which is that bit faster than the rest. That is Pds fault, not nenkais. Whether that means a lap or two difference in pit split, amount of fuel/tyre saving you should do, or if you should base your car choice off those parameters or other fine tuning is what it boils down to. I would argue not even the best players can get that bang on every time right away based on seeing the combo as its a matter of seconds, but race defining seconds. So yes people do need a bit of time to get that sorted, and a lot of that is now done before the races are launched. The second aspect here is just the aspect of practicing a combo and getting up to speed, which again takes a bit of time, and many now do that before races are launched. Thus there is less carnage in the first few hours and less variety in pace.
The best car can often be guessed now, but with relatively frequent bop and physics changes there are always times in the game where different cars need to be tested on a particular combo, but less of that happens in race.
And yes many people take one look at the combo and decide not to race, and when the combo is released obviously makes no difference in that case. But there's also a significant group of players who think, ok maybe this combo can be interesting, but then they see from testing before they officially go live that it's a one car, only one viable strategy type race, and decide they can't even be bothered on Monday instead of finding out when they jump in the race Monday. Or they jump in knowing the meta straight away. The bad race is Pds fault, the potential group of players that don't race because they have tested it or heard on the grapevine before Monday what it's really like, and the homogeneity of the first day of races are the leaks fault.
So yeah it's a little thing. Didn't think it would be a big deal, but lots of words have now been said about it. It's just interesting that there's such a moral high ground against leaking this list of cars or even hosting it on gtp, but leaking races is good because the 'pros outweigh the cons', even though it's altering PDs intended game design (whether you like the game design or not is a different matter)