I don't think it's at all obvious that Arthur wasn't a real person. We know, from both Gildas and archaeology, that the Britons managed to successfully resist Saxon advances from the late fifth to the mid sixth century. Someone must have led the British resistance of this period, and all the sources that name the British leader give his name as Arthur. I don't think it plausible that a fictional character would have displaced the real hero of the British resistance so completely that not even an alternative name for the victor of Mt. Badon would have survived.
Also, the more legendary/mythological accounts of Arthur generally postdate the historical sources by several centuries, and since Arthur generally plays a fairly minor role in them it's more likely that a famous historical figure got incorporated into pre-existing mythology than that all these historians chose to graft an obscure Welsh folk hero into their histories. We even have an example where we can see this process in action. Compare, for example, Nennius' ninth-century
Historia Brittonum on the settlement of Ireland:
After these came three sons of a Spanish soldier with thirty ships, each of which contained thirty wives; and having remained there during the space of a year, there appeared to them, in the middle of the sea, a tower of glass, the summit of which seemed covered with men, to whom they often spoke, but received no answer. At length they determined to besiege the tower; and after a year's preparation, advanced towards it, with the whole number of their ships, and all the women, one ship only excepted, which had been wrecked, and in which were thirty men, and as many women; but when all had disembarked on the shore which surrounded the tower, the sea opened and swallowed them up.
With the tenth-century
Preiddeu Annwfn:
Beyond the Glass Fortress they did not see
the valor of Arthur.
Six thousand men
stood upon the wall.
It was difficult
to speak
with their sentinel.
Three fullnesses of Prydwen
went with Arthur.
Except seven
none rose up
from the Fortress of Guts (Hindrance?).
Clearly the latter poem is referring to the same event as Nennius; but, whereas
Preiddeu has Arthur involved somehow (probably as the leader of the expedition), Nennius not only doesn't mention him, but sets the story in the mythical past, thousands of years before Arthur's birth.
One final point: the notions that Arthur had a court in Camelot, that his soldiers fought as heavy cavalry, and that he fought wars of conquest on the Continent, all date to the twelfth century or later, and are more plausibly explained as reflecting the social and political situation of high mediaeval Europe than as reflecting genuine historical or even legendary tradition. So theories which take one or more of these ideas as their starting point ("Camelot was actually Colchester, therefore Arthur was really Caractacus," "Riothamus fought on the Continent, therefore he was probably the inspiration for the Arthur stories," etc.) are likely to be wide of the mark.