Structured training helps for sure but I can also say that based on my personal experience, out of fatigue and tedium, one will reach his or her plateau much earlier than before.
Many reasons contribute to that too, like paralysis of analyses, not taking full data into account, unwillingness to de-load, nutrition, etc.
I have friends who ride out, with little to no structure but improved at a higher rate than when he restricted himself to indoor structured training sessions.
That said, structured trainings do aid in one thing, it is controlled stress thus lowers the chances of one getting injured. Also controlled stress equates to controlled fatigue, which is why many pros advocate the usage and subscription of Training Peaks to monitor one’s stress and that is another lengthy discussion that involves logarithms that mostly get derived from what you are exposed to right now, such as TSS(Training Stress Scores).
I do not monitor my progress to that depth and I am not informed enough to explain but I was taught and exposed by some silent readers here who are really smart.
I also set myself with weekly total TSS goal (450-550-450-250) to push myself further.. but just self research and self taught.. i wanted improvement only so just did some research and follow instinct.. because i got limited time to go outdoor for cycling.. need to split nicely between being a dad+IT support+cyclist..