I am a fervent believer in developers mapping out their games in advance, and it will certainly speed up the process. But I think you greatly minimize the process of putting out a release. For one thing, the content of each release cannot be plotted out in detail at the beginning. Real life tends to interfere. Then, developers always have fresh ideas as they go along that require rethinking and re-planning. You also left out the whole coding process, which is not an insignificant task, especially if there are multiple routes possible. The rendering itself is limited by the processing power under a developer's command. Some developers are working on potato computers, while others have so much power on hand that their main problem is cooling and their electric bill.
No offense meant. I believe you are right that proper advance planning will reduce development time, but it can't be the whole story of
Caribdis's fine progress.