I think there is no single reason for why long term plans don't work. It is more like you need to do this dozen things right for the plan to work. The longer the plan the more things you need to be doing right to be able to see through the plan.
Accomplishing short term plans can be done relatively easily. There is less chance something unexpected will happen that will throw your plan into disarray. You have a chance to "just focus and get it done". You can take on debt in order to accomplish the task.
The longer the plan, the better you have to be at dealing with various things that require you to have much wider view of whatever you are trying to accomplish.
For example, if your plan is to loose 3kg in one month, you can just do it pretty much on your own with little interference/inconvenience to your wife and kids.
Try loosing 30kg over course of a year and you start having many more problems that you might not appreciate before you took on the project. Maybe your family is unhappy with the change of the diet and your schedule? Maybe your wife is unhappy with your lower libido/irritability? If you want to keep your gains you need to learn to make your diet stick and change your eating habits, which is super hard.
In a company setting you might find that long term project might need you to rethink many more things like how do you keep people dedicated/motivated when it is hard to see the distant goal? How do you keep yourself motivated? How to deal with people leaving the project? How to deal with more urgent/short term project preempting the long term goal?
Setting aside if you’re right, you’re not addressing the third issue, which is as important, if not more, than the other two: “Assuming you know exactly what to build”
Also, among any other analogy you could have taken, choosing diet is pretty ironic (or perhaps completely apt actually?)
I think long term projects typically correspond with appropriately larger challenges. That's why I used losing weight as an example.
Even with losing weight, it is much easier to understand what you are getting into when you want to drop 3kg vs 30kg.
In my case feeling cold all the time for about a year wasn't something I have predicted when I started it. Or how difficult it is going to be to manage clothing department. Or to learn new recipes.
Now, none of the above would cause the project to fail, but one quarter into that year I figured out I need to find ways to keep me focused on the goal or I am going to fail. Just eating less wasn't doing it for me and I felt constantly hungry and miserable and I would soon drop out.
So I had to pivot and be innovative to be able to finish the project.
But... that's just the thing. You had a goal, not a long-term plan.
With that you would have set yourself with activities that stretched the span of your endeavors, say for half a year each week I lose 1 kg. I do that by eating this, exercising like that, etc. Then, things start falling apart with the hurdles you mentioned, and you adjust your plan by changing recipes, trying to find more fitting clothing, etc. So, you adjusted your long term plan to still reach your goal.
The point this article is trying to make is that you shouldn't stick to your plan but rather keep in mind your goal and change the plan along the way.
It is just an example. Simplification is a useful didactic/communication tool.
Project I normally work on span tens to thousands of people but they are usually very complex to give as examples without spending some more time thinking how to best explain it.
I am sorry that I caused you enough discomfort to point it out. I will try to do better next time. In the meantime I am still learning English -- not my native language.
I think there is no single reason for why long term plans don't work. It is more like you need to do this dozen things right for the plan to work. The longer the plan the more things you need to be doing right to be able to see through the plan.
Accomplishing short term plans can be done relatively easily. There is less chance something unexpected will happen that will throw your plan into disarray. You have a chance to "just focus and get it done". You can take on debt in order to accomplish the task.
The longer the plan, the better you have to be at dealing with various things that require you to have much wider view of whatever you are trying to accomplish.
For example, if your plan is to loose 3kg in one month, you can just do it pretty much on your own with little interference/inconvenience to your wife and kids.
Try loosing 30kg over course of a year and you start having many more problems that you might not appreciate before you took on the project. Maybe your family is unhappy with the change of the diet and your schedule? Maybe your wife is unhappy with your lower libido/irritability? If you want to keep your gains you need to learn to make your diet stick and change your eating habits, which is super hard.
In a company setting you might find that long term project might need you to rethink many more things like how do you keep people dedicated/motivated when it is hard to see the distant goal? How do you keep yourself motivated? How to deal with people leaving the project? How to deal with more urgent/short term project preempting the long term goal?