They should just make those books they are trialing in Legion useful for all professions. Possibly make them more expensive but then they restore all recipes that you have acquired before.
The benefits:
- Less need for profession alts, especially gathering "slaves"
- Perpetual gold sink implemented
- You still need to level / acquire recipes once, but not multiple times across different chars
- You can adjust to changes in professions without having to sacrifice rare / non-obtainable recipes any more
The downside:
- Some players might abuse the system, but that can be countered by the cost of the book
- Less need for profession alts might make people less inclined to sub through content draughts. Let's face it, they will continue to exist, no matter how much content is thrown at us in the beginning of an addon