The Prime Evils did not orchestrated their exile.
Book of Tyrael : I am of the belief that the Prime Evils quickly saw their exile as an unexpected and fortuitous chance to corrupt the mankind.
It means that the Lesser Evils defeated them.
“Tyrael was a fool to have trusted me! You see, it was I who told Diablo and his Brothers about the Soulstones and how to corrupt them. It was I who helped the Prime Evils mastermind their own exile to your world. The plan we set in motion so long ago cannot be stopped by any mortal agency. Hell itself is poised to spill forth into your world like a tidal wave of blood and nightmares. You and all your kind…are doomed.”
Izual told this to the player’s character in Diablo 2 after you struck down him down.
I often wonder how Diablo and his bretheren reacted upon reaching the mortal realm. It is entirely possible that, for a time , they cursed their fates. However, i am of the belief that the Prime Evils quickly saw their exile as an unespected and fortuitous chance to corrupt the hearts of mankind. - Book of Tyrael
Doubt it. Based on what Izual had told us in Diablo 2.
The Prime evils had always considered going into the mortal realm. The exile was just a means to achieve this.
Meanwhile, the book is totally describing things from the perspective of Tyrael alone.
This is something that Blizzard always likes to do: telling us something from one of the figures in that world instead of universal statement. This allows to further develop the character (as well as being a legit reason for retcons).
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In the book The Order. We can see the same.
Blizzcon - 2011 Chris Metzen told that Prime Evils had no chance , their own minios turned againts them.
we can read in book of Tyrael that Izual told us the truth in D2. But : ´´the Lesser Evils enacted a brilliant stratagem to ovetgrow the Primes. Led by Azmodan and Belial the rebel demons claimed dominion over much of the Burning Hells.,
After the Dark Exile in D2 only some of the demons served the Primes , the majority served the Lesser Evils.
Yes, they were overwhelmed by the forces of hell. But once again, that was all part of the prime evils grand plan as they wanted to get into the mortal realm.
Also, in Diablo 2, you had Andariel and Duriel leave hell to go assist the prime evils as a way to gain their favor, since they knew that the prime evils would return and retake hell. Meaning that the only demons who served Azmodan and Belial, were mostly Demons from their own realms.
Gameplanet: Of course! A final question that personally interests me – and I can’t hold a candle to Red Shirt Guy – but I’ve been curious: When the angels and demons learned of Sanctuary, they held a truce and agreed to a kind of non-interference pact. Obviously the Prime Evils had no interest in holding to that pact and their interference in Sanctuary triggered the uprising against them by the Lesser Evils. They lost that civil war and were exiled to Sanctuary, but that’s exactly where they wanted to be in the first place. Does The Book of Cain look to address or provide a plausible reason for that?
Metzen: There have been some theories that it was all by design. That tends to help that idea, that they did it on purpose. I don’t actually like those theories and although it’s a little funky – why in the world would these other demons…? The idea is that the Lesser Evils overthrew the Prime Evils because they were jealous of men, because they feared what they represented in the cosmic order. Ultimately they feared being replaced.
Whether he liked it or not, that’s pretty much established in the lore, that the prime evils wanted to get into the mortal realm, which is also reinforced by what Izual told us in Diablo 2.
Chris Metzen is a writer in diablo universe.
Chris Metzen , Book of Cain , Book of Tyrael - They are all wrong or Izual is a fool ?
Neither. He only stated that he doesn’t like the interpretation that the prime evils orchestrating their own exile, not that it is wrong. There’s a difference.
And why he doesnt like if it is true
I wouldn’t know, fact however is that Izual told us explicitly in Diablo 2:
And considering Tyrael completely believed Izual words, that means it’s true in my eyes.
Chris Metzen said ´´ those theories´´ not Facts 
And he dont like it.
Izual is lierWell considering Tyrael didn’t said the same in Diablo 2, I’d have to disagree.
If his truth didn’t make it into the game, then it’s wishes and ideas at this point.
Chris Metzen knows the truth
I know this is a nearly month old post but given the recent leak of Diablo 4 being likely to be announced, I came to the forums again. Anyway, I think it would be fair to say that Izual could have been fooled into thinking the Primes planned their own exile. I’m not saying that he was but it’s possible Izual was wrong. It makes sense a demon would lie to their subordinates if they legit lost. “It was all part of our plan.” is a very easy excuse. One that could be believed given their power and reputation.
That’s certainly possible, but from my understanding Izual had made his ill advised assault before the prime evils were even banished from hell. If that was the case, there wouldn’t be a need for the prime evils to lie.
There is one thing I dont understand.
How does the prime evils orchestrated the dark exile (you know, so they could corrupt the worldstone using the soulstones), IF Tyrael only created the soulstones AFTER the dark exile?? as a way to contain the prime evil that were free on sanctuary?
Like, how Izual would know about something that was not even created yet?
“The Prime Evils are gone. But even the Lesser Evils of the Burning Hells can destroy worlds, should they choose. Some say they are even more dangerous.”
More dangerous > more powerful