The Anglo-Saxon Chronicle (dubious as it may be sometimes), Gildas, and archeological evidence from Essex really don't agree with "fell hard". It very much appears that the empire didn't have enough resources to continue propping up a fringe border province from the death of Julian the Apostate onwards, and that Brittania brought in Anglo-Saxon foederati to help out, who filled a power vacuum after Constantine III left (and possibly invited others to settle).
Particularly under Honorious, fighting between the Vandals and the Franks tied up almost all of the Roman resources in Gaul, which mattered in a way Brittania did not, and the population started to move into fortified settlements, especially after Constantine III took most of the legionary troops off in rebellion. This isn't really that different from what was happening in Gaul.
Fifty years before the "fall" of the West, there was no meaningful military presence in Britain which was not foederati, and before that, the situation was so precarious that troops revolted, leading to its abandonment. As early as the crisis of the third century, Roman Britain's economy shifted to become far more regional, since long supply chains through Gaul were untenable.
The primary exports were extracted minerals. It was, at best, colonial exploitation in antiquity. Raids from the Caledonians had never stopped, ever. Britons convincingly repelled an Anglo-Saxon invasion in 490, and it was primarily peaceful after that (and before that). Balkanized, yes, but hardly subsistence living with roaming bands of invaders.
This isn't any different than northern Gaul, or southern Gaul as the Burgundians, Visigoths, Franks, and Vandals cycled through. Or Illyricum. It happened earlier, but not like you describe.
It's incredibly telling that Gildas had both the time and education to write a somewhat polemical history less than a century after a "complete collapse of the system, (where) people went back to sustenance living", in classical rhetorical style, wherein he mostly seems to regard Britons as still being Roman (despite the "fall" of the West by that time, and the abandonment of the province 100 years earlier), as a Christian, which also tells us that Christianity had not been abandoned yet.
This is utterly at odds with your views. Where and from whom did Gildas receive his education? Why should we disregard his accounts of Mons Badonicus? Why should we disregard the Anglo-Saxon Chronicle and archaelogical evidence from Essex, Wessex, and Kent?
This narrative is false.