It’s ludicrous to think of post-Roman Briton as just warring tribes, as though the Romans had been dog trainers holding back a pack of slavering hounds. The legions and magistrates left (or were expelled in the latter case), the people left behind were thoroughly Romanized Britons, not naked Picts or Asterix extras…
The Saxon kingdoms were established long before the Viking Age, and the Vikings that did settle (the Danelaw) never left.
How is this any different than what happened in e.g Rome itself?
As has been pointed out above, almost everything your write here is simply wrong.
The Romans didn’t go anyway. The Roman army was largely withdrawn but otherwise the population was unchanged.
There was no particular “vacuum” although the period between roughly 400 and 800 was probably a period of significant migrations. The degree of migration as a percentage of the population may be open to challenge from recent DNA studies but not the underlying theme.
The (sub-Roman) British population did not separate into ‘tribes’ - there were local Warlords who fought each other for supremacy but not on a ‘tribal’ basis. It was those Wars that encouraged the invitation of additional foreign mercenaries - you can call them “Saxons” for the purposes of this discussion but it was obviously more complex than that.
The Saxons then revolted against their British overlords and carved out small independent entities is Eastern and Southern Britain. There was then a period of relative stability with the majority of Britain still being governed by culturally sub-Roman British masters.
The villa and town culture were not abandoned or destroyed. They survive a couple of hundred years - in slow stagnation and decline due to the destruction of the sub-Roman economy, especially in trading, that was the basis of the culture. Then nobody had enough economic stake left in the culture then that faded too - but it was a very slow process.
There was a second major Saxon influx in the late 500’s which led to the destruction of most of the British states in England - and was the final blow to the town and villa culture. But that was dealt by the Saxons not the British inheritors and a couple of hundred years after the Romans let Briton slip away from the Empire. Given the chaos on the continent Britain was probably more ‘Roman’ than many other former provinces until these events.
After this period the seven main Anglo-Saxon states emerged and solidified - with the ‘new’ states generally absorbing the ‘old’ Saxon states from the first revolt.
British states still persisted, in Wales, Cornwall & Devon and Strathclyde for hundreds of years after that.
The ‘vikings’ raids did not really get serious until the late 700’s - destroying the Eastern Anglo-Saxon states until late coming Wessex managed to stop the rot.
Huh? Have people these days not heard of Alfred “the Great” and his town-building and literacy programs?
We even have a copy of his urban plans (the “Burghal Hildage”).
He’s not the first Anglo-Saxon monarch of significant note. There was also Offa, who had extensive dealings with Charlemagne - who offered to marry a son of his to one of Offa’s daughters (but old Charles got pissed when Offa countered that one of his sons should marry one of Charles’ daughters! This implied a level of equality that Charles found presumptuous - and was, as his empire was by far the more powerful).
Offa was no piker though - he built strategic fortifications to keep the Welsh out of Mercia (“Offa’s Dike”). He also established his own coinage.
The notion that Anglo-Saxon England was a “dark rock” totally cut off from Europe and languishing in barbarism simply isn’t historical.
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