To start with your question: My personal view, not supported by any serious research on my part, is that speakers of all languages can say anything and everything they might conceivably wish to say in their own language. If new concepts are introduced from outside, there may be a borrowing* of lexis, but the speaker's language will handle the grammar.
English grammar has been, and will continue to be, influenced by the grammar of other languages. AmE, for example, has been influenced by Yiddish, and that influence is having its effect on other dialects. But this is part of the 'same GRAMMATICAL ability to deal with nuances as other languages'.
The grammar of English has changed, and continues to change. To take one example, the progressive (continuous) aspect has been a comparatively recent addition to our range of tenses/aspects. A need was (not consciously!) felt, and the form appeared to satisfy that need.
The 2011 grammar of English will not be able to cope with the needs of the speakers of 2111. But, by the time we get to 2111, the 2111 grammar of English will be able to cope with it.
As you, rightly, point out, case endings in English have almost disappeared, as has the importance of grammatical (not natural) gender. Philo, if he joins this thread, will disagree, but I think that the subjunctive is 'decaying' in BrE.
I think that's enough for a first response.
* 'borrowing'. What a silly word. We are going to use it for a time and then hand it back?
(And notice the American/Yiddish influence on that last [asterisked] sentence.)
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