The only way a TA is becoming obsolete for those who depend on it is
(a) resists evolving to a point where an unmodded slow isnt going to land on anything (pretty unlikely - would be a bitch to balance)
(b) tying the probability of landing a spell on a mob of a particular level to spell level so that old spells become obsolete naturally (don't know why they havent done this allready)
The sad thing is that even if this happened there would
still be some people chain clicking their TAs.
Another thing they could do I guess is to tie spell effectiveness to the difference between mob and spell level. So, for example, a 50 spell landing on a level 80 mob would be subject to a 75% intrinsic mitigation of the effect, or something like that. That would allow spell sets to evolve naturally against content.
That said, you all allready know my opinion of the likelyhood of a combined spell line, so I won't repeat it.
I find the suggestion that this would wean people off their TAs amusing though
