Sorry, but English grammar is probably the simplest of any language. No gender. No declensions. Simpler verb conjugations.
For instance, compare the simple verb “to walk” in English and French:
English: I walk, you walk, he/she walks, we walk, you walk, they walk.
Two different verb forms.
French: je promène, tu promènes, Il/Elle promène, Nous promenons, Vous promenez, Ils/elles promènent.
Five different verb forms.
Even, “to be” – a verb that’s irregular in all languages – is easier in English:
I am, you are, he/she is, We are, you are, They are.
Three different forms.
French: Je suis, tu es, Il/Elle est, Nous sommes, vous êtes, Ils/Elles sont
Six different forms. You’ll find similar results from conjugating verbs in most other languages, too. Clearly, English grammar is easier.
Admittedly, spelling is an issue, but since most slang terms are easy to spell (it’s older words like through, cough, pneumonia, etc., that cause the trouble), you should be welcoming more slang into the language as a way to make spelling easier.
And how would leaving slang out of the language cut the learning time?
And as for the “long years of learning” – nearly everyone knows the rules of the language before they start school. The school is only teaching the names of rules that you already know and have internalized completely. You would easily understand why the sentence “I saw the movie we had discussed in class” is written with the tenses it has, even if you don’t know that “had discussed” is the past perfect tense (I didn’t – I had to look it up). Most grammar instruction in US schools is redundant and unnecessary in order to learn the language.
It already is adopted as a world-wide language. There was that memorable example in The Story of English (which I suggest you read before spouting off on this subject) of an Italian pilot, flying over Italy, and talking to an Italian control tower – and they’re speaking English. English is taught as the main second language in every non-English-speaking country in the world. It is, de facto, exactly the world-wide language you think it won’t be until it becomes “easier.”