Yes, the key part of estimation is not that we need to say how large must be the (time) box to contain the project, but rather how much of a project can we pack into a box no larger than what the business could bear.

Hence the separation into must-haves, highly desirable, and nice-to-haves. Hence the need for modularity and extensibility: you if don't get to build everything in one go, and can't always even predict what parts would be left outside the scope, you have more of a lego-like structure.

BTW maybe if we finally shook off the polite lie of planning how much work a project could be, and instead started to think in terms of possible deliverables within different time frames, the conversation would become saner.