I was always wondering. Did Goku and the other Saiyans still receive zenkais after the Frieza Saga. I think Akira Toriyama said no, but I'm not sure of the source.
I kinda assumed it had *some* effect during training, i.e. the harshness of training in the spirit room is part of why the SSJs were able to reach ascending/FP Super Saiyan, but it's effect was reduced.
It was really only explicitly around for a few stories, wasn't it? It happened left and right in the Frieza saga, but wasn't explicit before that point.
It's not the only idea that fell to the wayside, Kaio-ken being another. Though kaio-ken was a more interesting one, while Zenkai was just a way to catapult someone up the power fast.