They could make so that if you already have an instant 90 character on a license you cannot transfer another boosted character to that license. Or if you transferred an instant 90 you can no longer use the boost.
This will allow them to stick to their original ONE boost to 90 idea and avoid exploiting / the need to buy more games only for the boost.
Doesn't need to work like this and can probably be polished but this goes to show that there are ways to make it not happen, since there are quite a few arguments that people will buy more games and have instant 90's anyway.