Well Bruno, I guess WE are the biggest fools on the forum. I am in the same camp as you and as long as my bills are paid, I put the rest of my money towards my passions. I don't golf, smoke, or drink fancy wines so watches do it for me. I also live in NYC so having a car is not really a major factor in mind. I also do not have children nor am I married so that may lessen my "responsibility" load.
I believe you can afford the watch, but are intimidated by its price; I would be too!! I think you are considering this purchase in a responsible manner and weighing your current options and costs. Your kids will not be going to college for the next 15 years and that is a significant period of time. Not making any significant purchases today because of an event in 15 years, will literally tie your hands until then. I know there is a "what if" concern where money is good now, but may not be good in the future. Therefore at what price point is it okay to spend money without feeling burdened by that 15 year date? Is it okay to purchase a $200k car that will depreciate once it is driven off the dealer's lot, as opposed to a $400k watch that will hold its value and probably appreciate in time. Based on your net worth, job security and that 15 year due date, you will have time to save for college costs.
Although watches should not be treated as a realistic investment, Patek Philippe will probably hold its value better than other manufacture. With a grand complication, you have a super high end watch and if you need to liquidate it for immediate cash, you can enlist the services of a high-end watch auction company. The nice thing about these grand complications is the application process, especially if you need to sell the watch. Assuming you are approved and acquire the watch, you will pay MSRP based on its current value at delivery. On the secondary market, people may have the money but their application was NOT approved, so you can demand more money for them to acquire your watch. If you are acquiring the watch on the secondary market now, then you will probably get a good deal in light of the world's current financial situation.
Putting off this purchase until you are 50+ for no other reason than, "you should wait until you are older and worked more so you appreciate the watch more" is unrealistic in my mind. I purchased my first Patek at 25 and had to liquidate most of my collection to afford it. It was a difficult decision, but I do not regret it at all. I worked hard to acquire those watches, and therefore worked hard to acquire my first Patek too. If you have attained this success at your age, then reward yourself now. Maybe in the future you want something EVEN MORE complicated? I am looking forward to the purchase of a 5110 or older Calatrava in the future. I just need to keep saving and enjoy what I have. I would go for it.