To give some more context, I'm working in a medium-size, well-established company in its field.
The kind of problem we have when launching a new product is that we can have a well-defined list of everything that is required of the new product to be assured of successf - it's going to take 5 years to build that, with at least 20 people.
What can we cut and still have a successful product, that we can launch in say 1 year (or 6 months) with say 10 people? That is much harder to say, and different product leaders have different opinions, based on discussions with different clients. And the consensus opinion may simply change.
So sure, the architecture is generally driven towards that 5 year product idea, but the short term estimates are always going to be geared towards a specific subset of that. And that subset is easily subject to change, from one quarter to another. We won't throw away the design if that happens, but we will definitely throw away any longer-term estimates.
And charting out a 5-year plan that we would only 're-jig' as priorities change would be a recipe for certain failure - teams change, people leave, etc. You can't hope to have an accurate estimate on that time horizon.