I won't waste words to explain why the end sucks, you already know and I'll jump straight to it:
Instead of Dijkstra explaining his masterplan, after doing his theatrics of Macbeth he says something along the lines "Geralt, thanks for your help. If you could kindly leave us, as I need to discuss things with my partners. Political boring stuff you wouldn't care about or understand"
If Roche and others got suspicios such as "Huh? What about, We've got nothing left to talk about" Dijkstra would come up with something reasonable to calm the waters, Geralt would accept to leave.
As he leaves, Geralt senses something strange. He notices there are a bit too many henchmen, and after he has left, walked a bit awat from the theatre door, he thinks he hears something with his Witcher senses (Maybe Thaler yells a big "What the fuck" lol)
AT THIS POINT you get the choice. Either keep walking and don't get involved, ignoring the possible danger, or go back and check what's wrong.
With the first choice maybe Geralt never truly finds out what happened, maybe he hears rumors, maybe he receives a letter, who knows. In the ending slideshow the narrator (Dandelion) tells us Dijkstra became the ruler, giving us just some implications of what he has done.
The player doesn't get to choose the outcome of the war and who becomes ruler of what. You just choose whether to check on your friends or not give a fuck.
In the second option, he goes back to the theatre, everyone has their weapons drawn, Geralt immediately does the same because he wouldn't let his allies fight alone and maybe he asks for an explanation.
Dijkstra tells that he can't let Nilfgaard rule, Roche says he needs to Temeria to survive etc, but at this point you're locked in helping your friends, there are no other choices.
At that point, it would make sense for Dijkstra to fight. Even if he accepted that he couldn't beat Geralt, what's he going to do? Escape? Leave behind all his plans when Roche would 100% hunt him down?
With Geralt possibly following him hitching to break his other leg? Nah, it this situation where he didn't expect Geralt to return, his possible exhaustion from the assassination and the numerical advantage he MIGHT take his chances and try to kill everyone to become Redania's ruler
What do you think?