So does this mean that:
1. I have to pour in 15 more points into the weapon skill to get the original damage boost as before (with Tenebrium 5 only), or
2. If I put the 15 points, I will get additional damage boost?
Because if it's the 1st case, wow. Just wow.
Balance changes happen. It's a part of life.
The actual change is that weapon damage is now based on your weapon skill, and Tenebrium skill is required to wield higher-level Tenebrium weapons.
Admittedly though, that change is not very good because now you're going from "points into weapon skill do not matter for damage boosts, only Tenebrium does" to "points into Tenebrium do not boost damage only weapon skill does, but Tenebrium is required as a tax anyway".
Requiring a bunch more points that don't really do anything to help you is not great. It seems to me that would discourage using Tenebrium weapons at all.