Some years back I took on a embattled project, a dispenser/retriever for scrubs in a hospital environment that had a major revision stuck in dev hell for over 2 years. After auditing the state of the work I decided to discard everything. After that, we started with a clean slate of over 200 bugs w/ 45 features to be developed.

Product wanted it done in 6 months, to which I countered that the timeframe was highly unlikely no matter how many devs could be onboarded. We then proceeded to do weekly scope reduction meetings. After a month we got to a place where we comfortably felt a team of 5 could knock it out... ended up cutting the number of bugs down only marginally as stability was a core need, but the features were reduced to only 5.

Never once did I push back and say something wasn't a good idea, much of what happened was giving high level estimates, and if something was considered important enough, spending a few hours to a few days doing preliminary design work for a feature to better hone in on the effort. It was all details regarding difficultly/scope/risk to engender trust that the estimates were correct, and to let product pick and choose what were the most important things to address.