And the majority of development is (despite our egos) as mundane as this really. The effort is certainly NOT spent on design work; have you ever quantified the effort that goes into various aspects of delivering working production software?
Re. planning fallacy, what of it? It doesn't make estimating impossible; once you are aware of it, you account for it in your estimates. You also calibrate your estimates with independent models and with empiricism (i.e. previous experiences).
But the literature is extensive on this topic - I can't give a summary here but if you are interested, there are worse places to start than with Boehm; deeply unfashionable I know but built on (shock! horror!) empiricism and not the rationalist (in a epistemological sense) sophistry which dominates the thinking of "agile visionaries".
Really it's like we've regressed to a pre-scientific age in the field of software engineering processes. Most of the debate seems about as relevant as arguing about how many angels can dance on the head of a pin. It seem actually observing and measuring reality went out of fashion sometime in the 90s.