I think you are missing something: wanted features always exceeds time. You need some form of deadline to force your to think about what features to cut. Nobody wants to cut any feature that could make it in until forced to and so you are always late.

The deadlines are artificial, but once they exist they force a lot of real scheduling on many others and so they become real. In theory you can change them, but in practice people are depending on you.

Nothing is wrong with what you said, but keep it in context of the above.