From another thought-experiment-y perspective:

Say you have problem A to solve. Then either one of those is true:

1) it has been solved before, ergo by virtue of software having a zero cost of copying (contrary to, say, a nail, a car, or a bridge), so there is no actual problem to be solved.

2) it hasn't been solved before, ergo it is a new problem, and thus at any moment you may turn a stone and discover something that was not foreseen (whether they are rabbits, yaks, bikesheds, dragons, or what have you eldritch horrors) and thus of unknown cost.

Any task that cannot be obviously fit into one or the other can nonetheless be split into an assembly of both.

Thus any attempt at estimates is as futile as gambling to win, tasks are only ever done when they're done, and "successful estimators" are kings of retconning.

It's all make-believe.