A franchise, for lack of a better term, is an incarnation of "generation" of the Transformers brand. The word is used here to refer to the collection of not just toys, but also media, merchandise, and ideas that surround each of these incarnations. Because of these other elements, the word "toyline" would not be broad enough -- the toyline is just the toy component of the whole thing.

Transformers Cyberverse Franchise

Transformers Robots in Disguise 2015 Franchise