> hypothesis[1] is that English has been learned by foreigners multiple times in the past and slowly lost it's more difficult native features (noun declension, many forms of inflection).
The claim that noun declension and inflection is harder to learn isn't well founded. If your native language has these, it'll be more natural for you. All languages have semantic declension and inflection, in the sense that context signals the kind of each word even if it's not syntactically inflected. For example in English the position of the word can signal whether a word is an object or subject "John hit Sally" vs "Sally hit John", the order of John and Sally signals who hit whom. In an agglutinate language you could signal this by putting a prefix like "John hit Sallyum" or "Sallyum hit John" would be parsed the same. My native language is Turkish, I've been learning English since I was 5 years old, but this "order matters" thing eternally confused me. It doesn't make sense to me when people say suffixes are more confusing than this order thing.
The claim that noun declension and inflection is harder to learn isn't well founded. If your native language has these, it'll be more natural for you. All languages have semantic declension and inflection, in the sense that context signals the kind of each word even if it's not syntactically inflected. For example in English the position of the word can signal whether a word is an object or subject "John hit Sally" vs "Sally hit John", the order of John and Sally signals who hit whom. In an agglutinate language you could signal this by putting a prefix like "John hit Sallyum" or "Sallyum hit John" would be parsed the same. My native language is Turkish, I've been learning English since I was 5 years old, but this "order matters" thing eternally confused me. It doesn't make sense to me when people say suffixes are more confusing than this order thing.