Sort of don't need to. Japan continued G1 for several seasons after "The Rebirth" aired in the US. They created three major series "Headmasters", "Masterforce" and "Victory". As the years went along though, their series focused more and more on Japanese exclusive figures.
That said, if the series had continued in the US, I'm sure you're right - there would have been new technologies like Pretenders released. However, I think the tone of the stories may have changed as time went along from some of the cheesier elements of G1 (such as "The Girl who loved Powerglide" and Seaspray becoming a merman) to more "serious" fare such as the huge battles seen in "Rebirth" and the "deaths" from the movie.
As to which groups? If it was kept 2D animation - any and everything that had a toy.