disclaimer: have yet to play it on the new patch, GOG
That said, tenebrium reminded me of late Divine Divinity issues. Forget all your hard earned skills, now on you only boost the divine(?) (what were they called?) skill lines that just unlocked. Never got why they did that, but..
Casters-aside, tenebrium had similarities to said glaring issue back in DD. Will be honest and admit i was really surprised something even remotely similar to said concept had made its way back in. Partly the differences between the two, partly that playing two casters on hard sans any companions, ever, i kinda skipped by it.
I -do- however, personally, find it a wise move making it simply a prerequisite. Easier to maintain some semblance of balance, as you still have to measure and tweak the original skills, more logical within the RPG context, as it no longer pretty much invalidates all you have worked for thus far. If anything, i wish that, with DD in ther "CV", they would have had the insight to have eschewed it altogether. Honestly curious to know what got into them in the first place. This is a good move
(for those of you complaining, it's an intermediate, mini-balancing patch. You do not --need-- to run it..let it go. You want a new run, wait for the hardmode/balancing patch. Don't see any issue, why the fuss)