Well the problem is that you have to deal with the abilities somehow. If you keep them all then you are either asking players to go back to old content for current power, or just baking them in rendering the entire process of keeping them meaningless.
If you go for the latter option then you further might end up with the problem we had in MoP where all classes could do close to everything, and classes were extremely homogenous simply because the developers had given them something they didn't have previously over too long a time.
The system we have now, where whatever system either works only in the given expansions zones and content, or previously where it was somehow removed at the end of the expansion does work much better in the long term sadly.