if project realistically costs $100M and should take 3 years - it won't be approved due to budgetary and other political reasons.

Much easier to announce a project as $30M that can be done in 1 year, and intentionally skip over planning, and contingencies+complications.

Plan for naively simple project, like a Wordpress website, even if it is for is amazon.com.

When the project SNAFUs, go back to customer and use sunken cost fallacy to get more budget & time, until customer runs out of either Money (budget) or Patience (time), then complete the project and move on to the next "Grande Project"