That assumes that you know the unknowns and how to design for the flexibility you will need. That won't always be the case. Sometimes you just have to accept the fact that you don't know enough up front to make a good design, so your initial efforts need to be focused at gaining the information you need, for example by prototyping to get user feedback knowing that you will probably throw some prototypes away, not making design decisions up front that you don't have the information to make.
We agree. I just phrased it as design and plan for project flexibility, not design a bunch of knobs strictly for every case you happen to know about at some snapshot in time.
I say design and not just plan because you'll want to mock out or prototype something to show and that does take design work to keep your iteration cadence high.
In my mind flexible means easy to iterate on but maybe that is not clear.
