It doesn't need to include Saber either. UBW is a perfectly self-contained story without Saber being anything more than "girl who fights".
Except it's not. It was written to continue building upon the story the Fate route presented us. In UBW almost all of Saber's development happens offscreen or passively, as a result of her watching Shirou and Archer do their thing. This was fine in the VN, we had just spent an entire route following Saber and learning about her, after all. We didn't need to be shown the same development twice. However, this isn't following Fate, it's following Zero. They should have done something to remedy this problem, adding new scenes, modifying old ones, whatever. Instead, they dropped the ball and
decided to take the lazy path.
Additionally, Fate/Zero set up Saber as an interesting and important character, and they advertised it as a successor to Zero. Her change confuses many anime only viewers. In Fate/Stay Night, Saber has just returned from:
1: Having a generally shitty experience in the HGW, the only servant she actually liked was back stabbed in front of her.
2: Having her beliefs on what a king should act like be questioned and gaining serious doubts about it as a result
3: Finding out one of her most trusted knights went insane because of her actions
4: Being betrayed by her master at the very last second, right when her goal was within her grasp.
It's reasonable to expect that she would show some
reaction to this, but instead we get Saber being thrown out of focus completely, I don't remember her even reacting to Taiga's story about Shirou, instead she just kept eating or something. Nor can I recall any reaction to Shirou introducing himself to her with the surname "Emiya."
No, Sakura is not just a damn "sub-plot", she's important in Zero as well, and Saber is just there to fight for Kiritsugu anyway. There's absolutely no reason Saber should be given priority, and I'm astounded that you of all people are suggesting it.
In Zero, Sakura is Kariya's motivation. She's important, yes. But Saber is one of the main characters, or at least one of the most important member of the supporting cast. Furthermore, Saber already has a presence throughout UBW. Boosting her importance wouldn't have been hard, but they instead decided to use her for being cute in a fuckton of slice of life, screen time they could have used for new Illya scenes, or even to help deal with the disconnect Saber's character has.
On the other hand, to increase Sakura's role in UBW and resolve her story would require a great deal of effort. They'd have needed to write many new scenes, and modify many existing ones. It would have required alot of time and money, far more than with Saber.
And it's jarring for someone coming off of Zero to see Sakura's situation totally ignored as well. Doesn't mean they're going to cover it.
Not nearly as much, since she disappears very early on in UBW. I think so far Saber has had more screentime than even Rin.