essentially no grammar change is a "simplification". every language has the same complexity in it's grammar, it mostly depends where that complexity lies
you might just be confusing complexity with big words or large tables of grammatical affixes. over time, languages tend to either get more synthetic (closer to a 1=1 correspondence between words and morphemes) or agglutinative (many more morphemes per word), and it wraps around at the end - premium synthetic languages then start tacking words together, some start making other constructions, and suddenly you're on the agglutinative side!
Maybe... but some language requires more context for one to understand the utterance or sentence in order to give the statement the proper understanding. Why do some languages have a decrease in inflective grammatical cases (where English has now 3, whereas Hungarian retains 15 or more? And some languages are in the process of losing use of some cases; however, I'm unaware of languages gaining case complexity. Pidgins are simplified languages that allow communication between unintelligible languages. I would suggest they use simplified grammars.
Pidgins are almost a completely different thing. Since by their nature, every speaker of a pidgin is a fluent native speaker of at least one other language, the pidgin doesn't have to meet all of the communication needs of all of its speakers. Once they cross over into creoles eg have native speakers, they fall into the same structures as all other languages.
context is a part of language! pragmatics is essentially context as it relates to language, and pragmatics is a part of linguistics. languages gaining case complexity largely happens by affixation of prepositions - i can't think of any examples off the top of my head but you'll have to take my word on it occurring.
you might just be confusing complexity with big words or large tables of grammatical affixes. over time, languages tend to either get more synthetic (closer to a 1=1 correspondence between words and morphemes) or agglutinative (many more morphemes per word), and it wraps around at the end - premium synthetic languages then start tacking words together, some start making other constructions, and suddenly you're on the agglutinative side!