Design decisions are a set, not a series.

Design decisions are taken in the context of the design as a wole. That whole is subject to change throughout the design process. Therefore, logically speaking, all design decisions remain up for debate and themselves subject to change throughout the design process.

This is highly impractical, of course. In practice, therefore, important decisions are ‘frozen’ at some point during a design project. In practice, the reason for some decisions then becomes something like ‘because that follows from what we decided earlier’ or ‘it would cost too much to change’.