Social pressure and promotion

 

Normally, you would adopt a foreign language as your own if it provides a lot of added value. Suppose that all Britons spoke Brythonic before the coming of the Anglo-Saxons. What added value could the English language provide to them?

Roman society had been reasonably open. Roman law was applicable to every citizen. The nobility of one's family was not mandatory to access the highest ranks. Careers were very much possible.

Many Roman emperors came out of 'nowhere'. The British co-emperor Constantine III is such an example. He was qualified by his contemporaries as 'a man without nobility' ('sin nobilitas', or s.nob.). We have no idea from where he suddenly 'popped up'.

The knowledge of Latin was necessary if one wanted to make a career in the Roman administration, or in the Roman army or, later, in the Roman Church. Latin opened the doors to promotions. Speaking Latin in daily life meant that you had ambitions or that you had become a part of the upper-class.

In contrast, Anglo-Saxon society was completely closed. Power and rank was and remained a family matter. Climbing the social ladder, the ascent to power, was very limited. Birth determined what rank you had, not the knowledge of a language, not even knowledge in general. Such a social system was typical for the aristocratic age.

After Clovis had achieved his conquest of Gaul in 507 AD, Low Franconian, the ancestor of Flemish and Dutch, remained the language of the newly established Frankish aristocracy. The Gauls themselves continued their language transition to French.  Texts, contemporary of Charlemagne, reveal that the Frankish aristocracy had the habit of sending their sons to Brabant (Belgium) "to learn proper Frankish"[1]. Low Franconian was the language of Charlemagne's court. After the death of Charlemagne, the French aristocracy gradually adopted French as their native language. Low Franconian remained the aristocratic language in France for some 300 years (507 - 814 AD).

Speaking English as a daily language provided therefore little advantage. Why would the British change their language? This would improve by no means their daily situation. On the contrary, speaking English as if it was your own language, could make other people suspicious that you were misrepresenting your actual rank in society.

After the fall of the Roman Empire, (private) schooling was no longer fashionable. Only the Catholic Church would continue to teach Latin. In addition, there are no indications that the Anglo-Saxons organized some sort of English language courses. 

After the Arabs had conquered Egypt, they organised courses in Arab and Islam. The Muslims considered Arab to be the language of God. Attending to those schools was compulsory for the Egyptians. If you did not, you had to pay special taxes. This resulted in a language transition from Koptic to Arab.

The legal system during the Roman Empire encouraged a language transition.
The legal system during the Anglo-Saxon period discouraged a language transition.

The statement that the Anglo-Saxons spread English because they were in power can therefore be dismissed.