@TheGodEmperor Honestly, the issue with that is as you write, things change... even if you have an ending in mind, while you are writing a story, you got to try to fit it with that ending... and in the end, executing it would be difficult. I think often times, authors do have an idea of how they want to end a series... but, it has to fit the story you wrote... and as you write, things change. Humans aren't robots that use math to write a story that would just fill in from the beginning and the end... in fact, often times, authors try to get the original ending they wanted, but it doesn't work because of how the story happened.
And while a beginning is hard, if the beginning fails, then you don't continue the story.. it doesn't start... simple as that... and you can revise the beginning as much as you want before you finally release it... but a story that you write, while the train is moving is different.