There is more variety in how my native Norwegian is spoken, and that is only spoken by 5 million people.
You also don't seem to distinguish between dialects, written forms and a language. E.g. Norwegian is a language with dozens of dialects and four written form, two of which are official. The various dialects map to one written form more closely than another.
Nothing stops English from being a language of different dialects and different written forms. The point isn't how many written forms there are but that those that exist are standardized. That is IMHO not a very hard thing. E.g. the largest area where English is natively spoken is the US. And the US has almost no variation in how the language is spoken.
Yes I know American loudly object to this but I have traveled all over the US and lived several placed. Honestly there is not much difference between how somebody in Grand Forks, North Dakota speaks and somebody in Honolulu, Hawaii.
Yes, by American standards it may sound very different. But by say Swiss standards, Norwegian standards etc not very much. Even tiny Britain has more regional variation.
One would simply create American, British, Australian etc standardized written forms, while trying to harmonize each written form as well as possible.
I agree with your point in general that English as is spoken in international media isn't all that diverse, but if you look at the British Isles where the language has traditionally evolved for centuries you'll find a picture that more closely resembles other long-lived non-international (? national?) languages with rich dialectical variation.
Non-english languages are way better at this. In Slovenia, for example, we have about 2 million speakers. Those are divided into 32 dialects. Many of them mutually unintelligible.
https://en.wikipedia.org/wiki/Slovene_dialects
Germany has a similar problem with regional dialects.
That’s why these languages have a standard official version. Because they need one.
The fact English doesn’t have such a standard is fantastic evidence to the relative lack of strong dialects.
Then again, we do have BBC English and American Movie English which act as de facto standards.
Oh and air traffic control english is also heavily standardized from what I’ve heard. Defined with the rigor of an API.
What’s interesting about English is the relative lack of dialects outside the UK. It’s already a very standardized lingua franca because of how it spread.
And what’s left is solvable with the BBC/Hollywood English. Similar to how germany has hoch deutsch – an invented standard dialect everyone learns in school.
You can have a standard form of English taught in schools without demanding say-to-day stabdardization in common use. And we kinda already do. Just unofficially.