I think the idea is to force you to choose between the Attack bonus offered by using the planner, or the advantage you gain by using fine manual control. Both ways of controlling offer their own advantages so on paper, it's balanced and both play styles are viable.
If that's the idea, it is not reasonable. It doesn't matter even if the devs claim this, it's still a degenerate design decision and I doubt anybody can support it under rational scrutiny (there's a simple way to prove me wrong, but good luck with that one

).
The reason is two-fold:
- The planner has serious documented flaws, so attempting to compel players into using it is at best an attempt to mask poor AI by also giving player units poor AI.
- You can mimic "fine manual control" by simply updating orders
The latter completely upends the credibility of this mechanic change. Rather than right clicking a province, you can instead simply spearhead a province or three and hit "execute plan". Even if this path has nothing to do with whatever plan you had previously, you avoid the 8% decay. "Balanced" indeed. What pdox did here was strictly increase our number of inputs to move our units w/o penalty with no demonstrable design benefit conferred whatsoever (unless you consider gating the ability to do something resembling a right click w/o penalty behind DLC to be a "benefit").
The basic problem is that unlike the Attack bonus which is a clearly visible number, there's no easily quantifiable representation of the advantage manual control might give you.
In the current version of the game, if you spam plan updates the "advantage" manual control gives you is zero in game terms, just fewer clicks. If you play in MP without pauses the advantage is difficult to quantify...but it's asinine to "nerf" something by straight up increasing # of inputs and nothing else.
Anyway since you can spam new plans and doing so is the only way to retain the slow planning bonus decay...rather than hate micro pdox is forcing it down the players' throats if they don't want to take big penalties.
This is difficult for the wrong reasons. I'm reminded of old school Nintendo games with terrible controls. That's what this implementation alongside the battle planner amounts to doing...and it was done intentionally! That's mind boggling. Why not fix the broken mechanics first, then fix the planner, and not worry about players using their own inputs to work around the poor features?