[mchaver]

Languages often lose features but they also gain features. Complexity of language is hard to compare, but we can still find many examples.

Modern English has less complex verbal morphology and noun declension (as you mentioned, only in pronouns). But the set of vowels in Modern English is more complex than that of Old English. Also the vocabulary of Modern English has two main sources: Germanic words (native) and French/Latin/Greek words where a single idea can be expressed in either vocabulary source with different nuances. Old English was mostly comprised of Germanic words with some words borrowed from Latin.

Another interesting thing to note is that languages without tones can gain tones (tonogenesis) and in Old Chinese tones played a much smaller role than the modern descendants. This is often the result of syllables/sound systems becoming less complex and losing contrast so the tone of the word becomes contrastive to maintain a distinction between words.