We all understand the adaptations endurance rides give you, nobody in this thread is disputing that. I just think using the same term, fragile/brittle fitness, for why Jan Ulrich wasn’t more successful than he was and amateur athletes who pack their training plans with more intensity than they can handle is a bit stupid. By that token the every cyclist’s fitness is fragile once you set the bar high enough. The term suggests that the athlete is deficient when this is not necessarily the case. That’s why I don’t think it is a useful way to think about your training, and e. g. the mental model of an account with overdraft fees is better, because whether your fitness suffices is all a question about the demands you place on it.
Plus, I think there are instances where you want to put all eggs in one basket and peak only once in a year, but really, really high. An athlete training for the Olympics comes to mind: if the Olympics are my A++ event, then I might not care that the rest of the season is lackluster. Is that an example of fragile fitness or smart goal setting and execution?
PS Cycling isn’t the only example that comes to mind. Soccer had a similar scheduling problem a few years back when things like the Champions League were too close to the Euro and World Cups. Players couldn’t focus on one event well. Imagine being coached by several people with very different goals who want players to peak at different times …