How can you tell what is worth doing if you don't know how long it might take?

You make projections instead of estimates. You split the work that needs to be done into many tasks and project from past experiences.

You cannot rely 100% on any estimates either, and all you are doing by demanding estimates is creating stress and making people less productive. The meta work imposed by that in itself will make a project take more time, as everyone will be padding their estimates.

You work backwards. You decide how much time you’re willing to spend to get the worth. Then, take steps towards it with checkpoints.