The universe killed Giles Rosby because Cersei said that he'll outlive everyone in the room, and Cersei is never allowed to be right.

Gyles was last seen hoarse

13:52, what’s the old joke about Asoiaf theories? Benjen, Euron, Daario, Mance and Jaqen walk into a bar… he orders a beer.

Theory: Maegor the cruel killed gyles because he built the red keep faulty which caused Gyles' illness to get worse from asbestos or mould or some shit

I don't think Pycelle is opposing Cersei as much as he is practicing common sense and logic which Cersei opposes.

I'd guess GRRM killed Gyles Rosby

To be fair calling Olyvar Frey an "illborn wretch" would make sense considering nearly everybody despises or disdains the Freys after the Red Wedding. Not to mention it'd be greatly ironic (something George is a fan of) that one of the most benign and good Freys in the story be dragged down by something they had no hand in (and likely would've fought against it)

Maybe it's a pun. Cougher/coffer...one's dead, the other's empty...

Theory: Cersei killed Gyles by exacerbating his cough via inviting everyone to her big Hand’s Tower burning blowout bonanza. Whatever fumes alchemist fire give off can’t be good for the lungs and Rosby’s were already pretty bad, so the extra air pollution ended up tipping him over the edge.

In slightly more seriousness there is a parallel between Gyles living with his cough through most all of a long summer, but dying right before winter properly hits. A lot of super old people who lived longer than one would expect end up dying in the fall after their protracted illnesses reach a sudden conclusion, so it could just be reinforcing that theme.

I think that Cercei thinking something should be taken as evidence to the contrary.

The ward is actually Tyrek Lannister.. 🤯

"After the hanged man had stopped kicking, Vargo Hoat and Ser Harys embraced and kissed and swore to love each other always as Lord Tywin looked on."- CoK pg. 460. The fan art displayed at 4:34 is actually canon

It's very funny that the theory of Tommen being replaced by his cousin is effectively what happened on the show casting wise

Extra conspiracy theory: the Ward is actually one of Tywin's bastards, if we build off the theory that the tunnel to the brothel from the tower of the hand was built by Tywin (further compounding both Tywin's hypocrisy and Tyrion's assertion that he is exactly like his father 'writ small' and let's be honest, bedding Shae after her being with his son is something that would only be done by someone who is an extreme degenerate already). This would explain Pycelle's affection for the kid even if he isn't a Tommin double and if he is a double for Tommin then it explains why the ward looks so much like Tommin, and as you said with "illborn" only previously being applied to Joffery then it would be incredibly fitting for the term to describe another Lannister bastard.

7:06 commenting on a year old video, but Varys could just kill him quietly so that no one gets suspicious/overly paranoid. We hear about how pycelle wants guards that Kevan doesn’t give out because he believes they are safe. If Rosby was killed in the night, routinely going about with guards is a prudent choice and would leave less opportunity to take out the stabilizing members of the government. Assuming Gyles was killed at all.

Clearly Cersei overworked the poor bastard then goes and confiscates all the coffers of Rosby to fill her greedy pockets.

Why not Petyr Baelish again? If Gyles was appointed the master-of-coin, if he was competent or financially minded enough, he could discover that Petyr has been draining the Seven Kingdoms dry for a decade and a half, and Petyr can't allow that. And Petyr said to Sansa that he was struggling to form alliances quick enough to exploit Cersei's quickly collapsing regime, so maybe he poisoned him in coordination with the heir of Rosby or some other claimant, in order to gain an easy ally in the Crownlands.

It might be possible to slip an imposter past Cersei if it had been most of a year since she'd seen her son, during which time she had been drinking heavily.  Also, she was obsessed with Joffrey and I suspect never spent any time with Tommen - sort of he comes into the room, she waves him off while staring out of the window kind of "quality time".  And if she doesn't see a problem, nobody else is going to contradict her.

he climbed from just a member of court to master of coin by standing there and coughing... he played the game well until he was taken out who knows how far he could climb - picture the scene had he lived: Kevan has been Varys'd the realm needs a hand, Cersei scans around and hears a familiar cough, and sees Gyles "He'll do." she thinks, Gyles walks toward the throne coughing all the while before putting on the chain of the hand and imprisoning Cersei for all her crimes before taking the throne for himself. All hail King Gyles I of House Rosby. King of the Andals, the Rhoyner, and the First Men. Lord Protector of the 7 Kingdoms, and defender of the realm.
And peace is declared throughout...

And it would have happened if not for Qyburn and Cersei.

I absolutely love that title! Plus that Stretch Armstrong level of reach to make the final layer of the theory work is truly beautiful!