I think the focus is too much on what the Britons could do to stop the Saxons. The bigger issue I think is that the push/pull factors that fed into North Sea migration to being with. If the Saxons don't expand, the Angles and Jutes might in their place. A weaker Frankish Kingdom, for example, might allow for more overland migration and acculturation for the Saxons. In addition, the initial Saxon footholds could have been crushed by a more united effort of the Romano-British.
And that of course assumes that the migration model was more broad than the "warrior elite" model that many modern academics are increasingly in favor of.