'The decline of the British Empire should have resulted in the decline of the English language.' This statement is best described as

Saying the Empire's decline 'should have' shrunk English is a personal judgement, an opinion.

It offers a view about what ought to have followed, not a recorded fact.

English in fact spread after the Empire's decline, so the claim is not factual.

It carries no hostile pre-judgement, so it is not a prejudice either.

A reasoned personal belief about a matter is an opinion.