The chart below shows a chain of sound changes that happened to the English language, from 1400 onwards. Although the chart was intended to describe the Great Vowel Shift, it is not accurate*, since it describes not only the GVS, but also other changes, that happened later, and also affected the English vowels, such as [oʊ̯] > [əʊ̯].
As I understand, only the changes that affected the long vowels are considered part of the GVS. What is the reason for not including the other changes in this particular shift? In general, how do linguists determine when a certain chain shift is complete?
* Thank you, Aaron.