English and the protolanguages preceding it certainly were morphologically complex. English, though, has gone through a couple-three near-pidginizing events along the way that have made it a much simpler, more telegraphic language than it, in most ways, should be. Some aspects of the Celtic languages that were on the ground before English hit Great Britain have worked their way into the grammar (present progressive and do-support). The evidence seems to show that much of the inflective complexity was on its way out after the Vikings made the scene (the languages, Old Norse and Old English, were close enough to allow a degree of mutual comprehension, but differences in case markings, verb endings and the always-arbitrary grammatical gender made a hard-enough task more difficult than it needed to be). And one can hardly say that the influence of Norman French on the language was slight. Remember that the purpose of language is to communicate, so the long-term linguistic homogeneity of the group largely controls how complex the language can become. "City" languages tend to be much simpler (on average) than "forest" languages because outsiders rarely have to learn that 700-speaker language.

The surface grammar — what actually comes out of the speaker's mouth — doesn't directly reflect the underlying grammar/syntax, though. The 25-syllable Cree word will tree in pretty much the same way as the 18-word English sentence it replaces. The tonal variations a native English speaker uses convey the same information as the pragmatic particles a Mandarin speaker has to include at the end of every utterance. (We literates are often prone to forgetting that these squiggly marks are to language what choreographic notation is to dance or staff notation is to music.) We might not always be aware of the mechanics of including things like evidential marking, ergativity or pragmatics, but we do it nonetheless. And we've replaced (for the moment, at least) most of our tense and case markings with entire words. The syntax is structurally the same, we just add the word "from" instead of adding the ablative case ending. And really, does anybody actually need grammatical gender? (See Twain's The Awful German language for a reductio ad absurdum.)