But by raw number of languages standardising on English is probably not ideal. Probably some hybrid of Chinese grammar, Korean alphabet and Latin pronunciations would have been my pick for example.
You could hope for merging of communities that create an unified language, but whenever two communities are not completely merged, each will coin new words, the pronunciation will drift, etc.
That is because locutors create languages, not ruling bodies.
Examples of that include canadian french, and english as spoken in Europe, for which many words are not known to native english speakers.
Have some examples of these english words that native english speakers wouldn't know?
Even more of them can be found in business language, which did not become loan words.
Only occasionally is it used in a sense when a magnifying effect is implied.
You get a ruling body to declare a particular manner of speaking as the preferred one, mandate that it be taught to children in school and prevent anyone who speaks differently from appearing in mass media. After a generation, you have children who grew up speaking non-standard with their parents but standard with their peers in school. Many of them will speak standard with their children, so non-standard becomes something you use to speak to your grandparents. Then the grandparents die and almost everyone who speaks non-standard fluently doesn't use it regularly, so they forget. The next generation grows up barely aware that the standard way to speak is not the only way.
All of that doesn't completely stop natural language change from occurring, but it strongly constrains the range of possible variation. Locutors may create languages, but ruling bodies can tell them which language to create.
Because, being french, we actively do it, and it is an utter failure. People do not know the ruleset they were supposedly taught, new words largely come from the society as opposed to the institution, and thus the institution mostly acknowledges things after the fact.
From that pov, the thing you describe looks like a pipe dream.
Still, the descendants of emigrants who learnt the dialect of their parents can only understand the older generation, not their peers.
In a liberal democracy?
The Académie Française is responsible for defining the proper spelling of words and update the intricate grammar rules.
At some point the Éducation Nationale adopts the changes and requires that all children learn using the new rules.
Eventually style guides of major newspapers and TVs are updated as well.
This has worked mostly until the reform of the 1990s which is controversial. I think the Éducation has only recently started to enforce it (15 years later). Many publications still use or tolerate the old style. Microsoft office gives you two dictionaries to chose from.
So basically this has worked until people started to actively go against it...
And the language is explicitly standardized by the Accademia della Crusca.
Still you are not wrong, now that it is widely spoken, it is likely that the language will mutate quickly (and signs are already visible).
Orthography becomes irrelevant as soon as you use Hangul to write a language entirely different from Korean, which wouldn't have the same inflected forms.