I think it was adding insult to injury at that point. She's already on poor terms with Paul before the confrontation with the Emperor, then he puts the cherry on top by pursuing Irulan.
Villeneuve needed to create a hook for Dune Messiah. The ending of Dune the novel ( to be fair, it was written in the mid 60s, and Herbert was influenced by the contemporary geopolitical culture, tone and struggle at the time, it's hard to hit him too hard for concepts that now seem dated or unwieldly) is different from the ending of Dune 2 from a strategic political viewpoint. A contested rule is simply far easier to write a sequel, under which Villeneuve had no guarantee would happen.
Pugh was cast because she's proven before that she could carry Chalamet. The Paul/Irulan dynamic is an almost direct port over from Little Women. Pugh is phenomenal in that film. She steals every scene and carries the entire movie on her back. It's not a huge secret the industry is attempting to create a new normal with masculine female characters and soft cornered Beta male characters with patently reserved feminine qualities. Zendaya can only play herself, she has no acting range. But her general tone is overtly masculine. Pugh is also masculine in tone, but she's got an edge of vulnerability that you can't teach, you can't manipulate and you can't fake. Pugh at least attempts to balance out Chalamet and Paul. She's not trying to sell why Paul should love Irulan, but that a potential third film can relate to the audience by making it a more than reasonable question in the first place.
While Dune "canon" has cornered Villeneuve in so many places, here is a character dynamic that already exists that has a lot of built in conflict already. A large thematic driver of Dune is the idea of free will. It's why religion and history is so infused in the work. Paul has to watch his father marry for power and duty, but also have a concubine / "other" in Lady Jessica. Now he must do the same to his own future children. A formal wife, who he doesn't know if she will resent him for the arrangement, and an "other" who carries all of the baggage but none of the privilege of being the biggest political target in their universe.
Can you sell the "Mentat" dynamic to a casual viewing audience? A human super computer? A cross between Bill Gates and Tom Hagen? Well you can't really so the Dune films doesn't try too hard to go there.
What can you sell to a casual viewing audience?
Here's something that's politically incorrect to say, but it's the truth - Sometimes in life, you are into someone else, maybe you even love them, and your feelings are no longer rational. They trigger you in ways you cannot begin to understand. You just want them. Now with people like that, if you love them and they don't want you back at all, you get older and you understand that this is how life works. It might hurt to some level, or be disappointing, or be a point of self reflection that isn't pleasant to face, but it's how the "game" is played.
But try the alternative.
You love someone completely. And they only want you just enough to pass the time. You are a reasonable proxy for them for now. Transactional. Almost mercenary with a smile. And the best you might get is to be settled for and resented for it in silence. That right there ( essentially what drove Eternal Sunshine Of The Spotless Mind) , that's the kind of dynamic that will utterly break some people in half. That's the kind of emotional devastation that some people might carry the rest of their lives.
If I am trying to build a script to draw in a casual audience for a film, I can sell the above. I can attempt to get them to relate to a very unpleasant but not uncommon situation regarding the basic human condition ( though the lense of an arranged marriage). However I can't really sell a Mentat to most people.
Villeneuve is a good filmmaker. He's attempting to set a Paul/Irulan/Chani dynamic that doesn't edge completely outside of some relatable convention for a third film. In a deeper question of destiny versus free will, what if your destiny is to surrender your own free will so others can have it. But you can't be sure because your father thought the exact same thing and all it did was trap you into the same vicious cycle.
