This question has already been asked numerous times and you're probably gonna get some shit from some users (most probably swerzy lol) but I'm gonna try to answer your question the way I see it.

So when the creators decided to span this game over 25 years, they probably realized that they will not be able to bring back the OG human characters for many future games since they age normally and logically they won't be able to fight as they did back in their day.
Well there are some very old characters in most fighting games (Wang, Gen Fu, BRC, etc) but they mostly fill in the role of the "old Miyagi-kinda mentor" fighter. Characters like Sonya, Johnny and Jax wouldn't fit that stereotype well so they probably wanted to use them while they could. If MK11 spans over the course of another 25 years that could not be possible for sure.

So that's where Cassie, Jacqui, Kung Jin, etc come to play. They are supposed to be the descendants of the human heroes but the creators learned from their mistakes in the 3D era games. They brought in newbies that were supposed to replace the old characters and gave them their moves which pissed off the fans. The reason was mostly because characters like Kira had no reason to borrow such similar moves to characters like Sonya and Kano, let alone replace them in the roster. Cassie and Jacqui are direct descendants of old characters so it would make sense for them to play similarly. The fact that their parents are in the game too, was a decision that prevented the backlash they got from the fans in the past. It's a way to make the "transition" to the new era, a bit smoother let's say. At least that's how I see it.

So I would guess that they wanted the fans to warm up to the newbies first, and in the next game have them actually replace the old characters. We may not like it and we may have wanted characters like Fujin, Havik, Reiko, Li Mei, Frost, Ashrah, etc, but I can see where they're coming from.

I agree with OP....65% of the roster is worthless

Well that is a surprising statement especially coming from you, I don't remember you ever mentioning your distaste for the roster before.

The OP can't even be bothered to explain why he/she doesn't like them. Heh.

Yeah continue to whine like a broken record. Guess we gonna be hearing those words until MK 11

depends if the roster is good or repetitive bullshit