I guess it depends on what you see as realistic. QOS does try to be a far more gritty and realistic spy thriller as far as the story and plot go. The whole idea of business interests working hand in hand with the intelligence communities and manipulating governments and staging coups, assassinations and terrorism to further their agenda is a sad fact of the way the world works. And the truth is most of these kinds of people really are above the law. Take Haines for instance, supposedly they got the goods on Haines, they know who he is, which incidentally was the reason White was going to kill Haines in the deleted scene however since it was cut I don't think it would or should be considered cannon. I find it interesting that Craig's suit and the production designs from the Skyfall PTS and later at the inquiry shooting are very much like the look from the few photos and footage from the deleted sequence. Almost as if now they can kind of pretend that the scene never existed. I think the reason why it was dropped was because that ending with everything tied up nice and neat would have been too unrealistic.
As it is now the last time Haines is referenced in QOS is during the first meeting with Mathis where he tells Bond to destroy the photo of Haines, almost as if implying that he's untouchable. The man is a close advisor to the then current PM, Bond was accused of killing his bodyguard, all they have are photos and hearsay from Bond, Greene is dead so no way to confirm what he said and we are never told or shown what Yousef reveals. Most likely things ending where they did at the end of QOS, in a sort of draw, stalemate. They can't realistically go after Haines without causing a scandal and even then he could always and most certainly claim plausible deniability if accused and of course he would use his influence to launch an investigation into the shooting of his bodyguard and bring M into public and also disclosing that MI6 are using assassins... yeah that would all kinds of scandals so realistically QOS ended in a draw and Quantum if they were smart would go underground and bide their time, perhaps even orchestrate some kind of revenge.
And then in Skyfall right off the bat MI6 and M are under attack not only from external threat but from assault by the bureaucracy that they are sworn to protect. That to me implies some kind of fallout from the battles with the all-powerful Quantum. Haines was a close advisor to the PM, now all the public inquiry, witchhunt and scapegoating and changing of the power structure seem even far less contrived and make sense. You'd be surprised that more often than not usually when there's some kind of real life breach of information, it's usually the fallout of some kind of internal power struggle in the organization or political structure governing it. Very few things happen by accident. Silva mentions rigging elections in Uganda "all to the highest bidder". We saw White in Uganda doing business with LRA so we know that Quantum has some kind of interest in the country so it wouldn't be a stretch to imagine that perhaps Quantum or individuals affiliated with the cabal were in contact with Silva for his services at some point or another, perhaps even unwittingly on Silva's part. Silva must have had some powerful backers/sponsors and I don't think it was the Chinese. They don't need to outright spell it out or even reveal if they were or not but the fact remains at the end of the events in Skyfall, Bond's greatest ally in his fight against Quantum, M, is dead.
With Bond 24 and the possibility of Quantum returning I feel the whole thing shouldn't be Bond battling Quantum and/or White. If anything The PTS should be stand alone and then perhaps the first act could seemingly be business as usual, Bond, M, Q, gadgets, Moneypenny, the mission established then they could have a bit of mystery and revelation that it's really the remnants of Quantum lead by White trying one last scheme to kill Bond, discredit MI6 and gain a foothold again only for White to meet his ultimate demise and Haines to be caught red handed. Like Bond says to Mathis, they say you're judged by the strength of your enemies, and to have these said enemies just fade away and not be dealt with proper just doesn't seem right to me. Even more so after Skyfall. I do agree when you say that if Quantum returns it would not be in exactly the same form we saw them in. If anything just a shell of the former cabal. Unlike some, I feel that we've seen all there is to Quantum. For the most part the folks at the opera were the Quantum Cabal. Maybe we might see some new faces if they bring them back but for the most part White is the key. He's not Blofeld, but he's essentially that kind of recurring villain I feel. He's his own character.
At the very least, if they really go the route of saying that Bond destroyed Quantum between QOS and SF than at the very least I would want them to bring back the character of White and give him proper closure. Bond needs to kill White proper. Even if it's not Jesper Christiansen. They could always recast and say that the Character had plastic surgery after the events of QOS, which would be very realistic seeing as they have his face and with facial recognition software the way it is now. And most likely White would change his name and identity in the years between the stories. At the very least there's a case to bring back White for closure. His characters ultimate fate effects not just QOS but the ending of CR as well. You could arguably say that Blofeld's organization SPECTRE was only ever an actual threat in TB the novel, in OHMSS and YOLT it was just Blofeld and not SPECTRE that Bond was after, infact if I'm not mistaken they make reference to SPECTRE being dismantled between the events of TB- TSWLM- OHMSS in the novel continuity. So SPECTRE, the big bad original terror group met it's end off page so to speak in the original novels so haha. We still got ultimate confrontation with Blofeld, some thing that the films really messed up. You see I don't want Mr. White to turn into Blofeld from the FYEO PTS. I want him to get a Blofeld from YOLT novel kinda send off.