You are basing this idea on limited play-styles and a narrow view.
There's basically two ends of a spectrum that matter. Organized and disorganized. The former roflstomps everything, the latter has the possibility of failing no matter what you do. However, what looks disorg isn't always actually disorg.
More than one group of enemies can be pulled at once.
Done it. Not specifically associated with either Org/Disorg. Easy to wipe or be wiped by the whole map at once.
Archers and other DPS units can do enough sustained damage that makes every point of Barrier count.
They really, really, do not. They attack really really slowly (relatively speaking), and you'll lose more barrier to decay than anything else. Players can readily cast barrier in between attacks, or let your teammates eat an arrow or two under normal circumstances between barriers.
If the player is skilled at recast, then it is better to have a fresh barrier. If not, then KP + CD is better.
I've also spent a lot of time just dodging stuff, watching weapon animations and how everything targets. The game is really really generous with burst damage.
Fact is that the decay rate reduction means
Don't think your one size fits all model is correct either. Internal logic can be correct while the problem is incorrectly modeled and when the base assumptions are incorrect. 1 + 1 = 10 , when you assume binary's base two instead of base ten.
Decay rate can also trick players into thinking the barrier will last longer than it will when burst damage occurs. It is a skill based meta thing, but it occurs. When you change the base assumption that the player is not you, then the whole problem changes too.