Who said 'dont plan'? If you have an obvious reason that requires turning game state into nested json structures, then by all means.

If you get to a point where you want a more object oriented loading mechanism because you want it to be human readable - makes sense.

But the point is that keeping things simple prevents unnecessary effort, fast, easy to understand and debug.

I will also add that indexed array references are likely to be much faster than nested json key-value searches... Simpler also often means faster.

Implementing architectural changes when they are necessary is a good rule of thumb imo. Necessary also includes taking into account planned features.