Interesting, I didn't know that. There was a big period of confusion in the overlap with some keeping to patronymics, some using as a family name. I'm not sure if it was Christianity, or personal preference that made the switch, as Christianity wasn't fully rolled out to England yet, again Danelaw areas being late adopters. Parish records of births, marriages and deaths were still 500 years away, so maybe no one really knows.

Helpfully, the old Saxon custom of the eldest having a particular first name - which would be given to the next eldest if the older one inconveniently died, stuck around a good while too!

How often were sons named for their fathers? I imagine it would be confusing to have generations of John Johnsons around.

Regarding “the eldest having a particular first name which would be given to the next eldest if the older one inconveniently died”—is that referring mainly to childhood mortality? I’ve read of names being reused at times due to the (by modern standards) shocking rate of infant mortality but I’ve never heard of that particular custom.

I honestly don't know, but probably not that uncommon as naming for the father has carried into the modern era. No less confusing to have three generations of John Smith.

The eldest getting a given name may have been a status thing, perhaps tied in with succession. Eldest being the one that would inherit the land or title. The young had all sorts of opportunities to die, and even escaping childhood was no guarantee as you'd then be expected to make up a Lord or King's retinue of men for any conflict. Maybe it's the origin of popes and some monarchs taking a name on succession - the "job name".