Hmmm.
Well I could definitely see bickering between the Champagne-Blois and Dreux-Courtenay lines over the regency and who are the closest heir lines. That might destablise things initially.
However if Louis dies then one or the other would centralise authority under them and use that to centralise France.
The best way to decentralise France is to maintain Louis until he has an underage successor followed by a weak one. This should reduce the King of France with the Dukes of France in a similar position to the HR Emperor and Electors.
So would this work?
Philip dies before Louis is of age.
Louis tries and fails to break the Angevins and other issues in the way of strengthening Capet authority/centralization, much as Frederick II OTL tried and failed to deal with those of the HRE. Much of the "regalia' powers (Not sure if this is quite the same in France, but it'll do for this point) are secured by nobles in an attempt to keep their support of his efforts in the South and against the Angevins. His son takes over, but dies after only a few years, without a son of his own.
In an attempt to resolve who takes over (I'm going to quote Faeelin's timeline, though the heresies aren't an issue for mine.) after the main line of the Capets goes extinct, something like this happens:
It had been the Count of Flanders who had, in the end, proposed a simple solution. The Capetians had been chosen to be the kings of France by an election; why, proposed the Count, could the peers of France not hold another one?
So, after much wrangling over which lords could vote, whether or not being a member of a heresy such as the Waldensians or Franciscans excluded one from voting, and where exactly the election could be held, the great nobles of France had come to Paris, with small armies in tow.
The election could be held, and now the main candidates sat expectantly as they waited for the Count of Flanders to announce the winner.
The Count looked around the room. There was the Capetian boy, Phillip, ready to be a 2nd Charlemagne. The Count of Dreux stood there impassive, not giving away anything. And the Count of Champagne, the last of the great lords who claimed the throne, sat there with a flushed face and acted as if he’d drunk too much of the wine which made his county famous.
“Are we ready or not?” demanded the Count of Champagne. “Who won?”
The Count of Flanders repressed the urge to laugh. “As far as I can tell, Phillip received four tenths of the votes. The Count of Dreux received three tenths, as did the Count of Champagne.”
Phillip smiled. “It is so good to see that my loyal vassals recognize my rights. I, Phillip, King of France, do hereby declare that-“
Phillip was cut off by the Count of Dreux. “Wait a minute here,” he said. “I challenge the validity of this election.”
“What?” said Phillip. “On what grounds?”
The Count of Champagne also protested, slurring his words as he did so. “Rigged!” he said. “The electors were bribed!”
”By you, maybe,” said Phillip. “But how dare you accuse me of interfering in a holy process to confirm that I am your lawful sovereign?”
“How can we trust you?” demanded the Count of Dreux. “Everyone knows you’re in bed with the damned Waldensians. I’d sooner trust a Saracen than you.” The Count reached for his sword, before realizing that it was not there. “I refuse to recognize the results of this election, and you can be sure that none of my supporters will either!”
“Same here!” said the Count of Champagne, who staggered out of his chair. “In fact,” he said, “I demand a recount!”
Phillip drummed his fingers on the table. “You jest.”
“No more than usual,” replied the Count of Champagne. With that, he left the room.
Phillip soon followed after, swearing that the election had merely confirmed what everyone already knew, and that even without it he was the lawful sovereign of France.
The Count of Flanders, who remained behind, thought. The war was going to continue, until God chose to call the contenders to him.
The Count finished off a pitcher of wine as he sat there alone. There were worse things, for a count such as him, than weak and quarreling kings.
Poor France, so far from God, so close to everyone. The interregnum would last until the 1270’s, irreparably damaging the monarchy in France. In Aquitaine would emerge great and free cities, which fought their lords for privileges and won. Normandy and Champagne would remain independent principalities, and the Count of Flanders would, in 14th century, become the lord of much of northern and eastern France. France would spend the next few centuries as a battleground for the armies of the powers of Europe, ravaged by the English, the Germans, the Aragonese, and the Castillians. The crown of France would eventually pass to the Counts of Flanders, but it would in truth be almost worthless.
Though of course in this timeline, Normandy, Anjou, and Brittany are part of the Angevin Empire, rather than independent.
I don't know if that unfolding is plausible in the posited scenario involving Philip dying young (he'd be in his early-mid thirties), though. France seems a little better off even before Philip than the HRE, but it doesn't seem impossible...thus running it by those who know more than I do.
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