When knowledge is limited, and the observable sample is small, it is only by extrapolation in accordance with the principle of Inertia that we can theorize about both the future and the past. Change is not a natural action but an exception requiring the introduction of some force. Unless the existence of a changing force can be demonstrated, Inertia dictates what conclusions we are alowed. We can't just make up the past out of whole cloth.
Working backward through time, if one argues the English were once Celts or that the English were once Picts, you need a force to enter the equasion by which these Picts or Celts might be transformed into English. Orthodoxy realizes this of course, so they appeal to the Anglo-Saxon invasion as the catalist for change. Harper demonstrates however, that this force of foriegn invasion, in every comparable situation where direct observation is possible, has proven inadequate to the task of effecting linguistic change. Orthodoxy must make of the Anglo-Saxon invasion an exception -- so they do so, but without any evidence to demonstrate in what way this invasion might be characterized as exceptional. We have nothing to indicate that the Saxon invasion was any different from the Roman, Danish or Norman conquests (as three specifically British examples) which effected no such change in the spoken language.
Having failed to demonstrate that the Saxon invasion was capable of altering the linguistic landscape of the British Isles, we are left with no choice but to leave Inertia undisturbed -- and conclude that the English were always English.