English is complicated because it's decentralized and there is no authority to regularize it. Which is a feature, not a bug.
1 - Being fluent in the national language does not prevent people from maintaining their dialects in parallel.
2 - Whether a language is phonetic has no relation to political issues concerning dialects.
3 - Whether a language is phonetic has no relation to whether people like to use it.
4 - English got decentralized starting with the Age of Sail, but the lack of correspondence between written and oral forms is systemic and older than that.
That's not really true -- there is and was a great deal of dialect diversity within England itself. It was widespread printing that allowed languages to be standardized at the scale of nation-states in the first place: the divergences that developed after the age of sail were reversing convergence that had only begun a couple of hundred years earlier.
And although versions of English from the south and east of England became the basis for modern standard English, other dialects persisted and sometimes spread around the world, so some of the differences between English dialects globally are due to disparate influences from different dialects originating within the British Isles.
e.g. Northern Italian languages are technically more closely related to Gallo-Romance languages from the other side of the Alps than to standard Italian.