As stated, my search for a previous thread with the answer came up with nothing, but this question FEELS like the devs might hear it as often as Ozzy has to retell the bat story, So I'm thinking either you hear it so often that everyone ignores it now or it's like the crazy old aunt living in the cellar that everyone knows is there but nobody wants to talk about.
Why can't Characteristics be increased with experience after character creation?
I know that characters get one point at Tier 4 of their skill tree, so do you have to multiclass if you ever want a second point? It smacks of artificially hobbling PCs. If that's what you were going for, well done.
If you just wanted to make it really hard, though, why didn't you just make it REALLY expensive? If it were, to pull a number from somewhere behind me, New Level x 100, by the time a character got to "overpowered" levels, chances are the NPCs would have been escalated as well and a character who had somehow ended up with all 6s would likely have had enough adventures and an exciting enough campaign to have earned them.