Changing world, can we afford to stay neutral and plan long term?

There is a dilemma for the project managers. One hand they are expected to have long term plan for the projects and on other hand, there is nothing like constant so it has to be dealt with flexible system. Though there are always check points to get the feedback of the project and then fill them back into the system so that the system can generate some responsive strategy.
Sometime we get so much confused with this dilemma that we start asking ourselves that when we have so much uncertainty, why plan at first hand. This is true that we need to modify our plan according to the changing demands of the environment and shareholders but the general project outline gives a framework to work in. This framework gives a direction to proceed. Other important thing to remember is if there are lots of changes in the project that also mean that the project was not planned properly or the project is not relevant any more. We should sit back and analyse if the project is still relevant. Many a time, we keep working on projects that are not relevant because we find it difficult to forget the sunk cost.
We should also be clear in our mind that it is not always possible to plan everything in advance because either we don’t have expertise to do so or it is too expensive or time consuming. We need project plan with some realistic goals.