And I would argue that its ultimate impracticability is one of the main sources of the rise of feudalism, as already expounded above. The king and his army can't be everywhere at once; he cannot intimidate everyone by himself, so he is forced to use intermediaries.
But he is not forced to mortgage his long term security in order to secure the services of those intermediaries. Otherwise, every state that has ever existed would have been feudal
Look, the bottom-line is, there is no way for an early medieval monarchy to remain strong and centralised.
One hundred and fifty years is not long enough to count?
You are yet to give any convicing arguments that could not be given for OTL strong centralised early medieval monarchies that still DID undergo a prolonged period of feudalism after that in spite of said arguments.
But you have not given any convincing arguments as to why an unconquered England should be much more feudal and much less centralized and stable, and possess much less national loyalty than OTL England.
His army is weakened and tired, and major preparations are needed for a proper campaign.
No, not really. You just get some ships together, not that hard, and go live off the land. Harold doesn't even have to worry about shipping warhorses across, which was one of William's bigger problems.
Furthermore, you are making rather unjustified presumptions aobut the extent of Norman weakness.
I'll go through my thought process then. First, I am assuming that William was defeated in a battle somewhat similar to OTL Hastings, except further inland and with more soldiers, particularly archers, on the English side. When William's Bretons break, the English maintain discipline, so the Norman charge breaks on the shield wall. Seeing this, the right wing joins the Bretons in flight. The Normans in the center stand, holding just long enough to be surrounded by overwhelming numbers of English. They are subsequently almost totally wiped out, including William himself. His army loses whatever sense of cohesion it had and begins a panicked rush to the boats, with Harold in pursuit. Large numbers of Norman soldiers are lost and rounded up by the English, the natives turn on the invaders, and in all the confusion, only a small fraction of the Normans manage to get on the boats and get away. Considering Hastings, where Harold's death led to a rout and a significant proportion of the English nobility died on the field, the scenario seems reasonable.
Now, William's force was about seven or eight thousand men, approximately equal with the English forces at Hastings. However, while that was rather less than England's full strength, the Norman force was significantly more than existed in Normandy itself. A couple thousand were Norman, representing pretty much everything Normandy could muster, while the rest were Breton and Flemish mercenaries and poor knights from all over northern France. William couldn't pay them all with money, so he of course had to promise land after a successful conquest. With his defeat, his army gets nothing, except a Saxon axe between the eyes. I doubt that many people would listen to Norman promises after such a debacle. Therefore, William's successor would be forced to defend with only the resources of Normandy itself. However, the majority of the Norman army went to England, including all the best troops and most of the top nobility, and most of them died there. It therefore seems pretty reasonable to think that Harold could take perhaps ten thousand men to Normandy against three or four thousand Normans, at most.
I must also add that I still fail to see any real purpose for such a senseless foreign adventure.
Settling a long held grudge, ensuring that Normandy won't ever threaten Harold again, conquering a very rich, populous and urbanized area, and securing the Channel against any possible naval attack. Apart from anything else, ensuring that Normandy was in friendly hands was the crucial aspect of English foreign policy for about seventy years. That's why William was in a position to make a claim.
