The premise of this question is a misconception. I will approach this from a more linguistic background. Languages do not tend to become less complex over time; parts of languages certainly do often show this trend, though not as a rule. However, complexity in grammar is there to signal meaning, so when one part of a language loses a particular complexity, another part of that language can shift the burden of providing meaning onto its back. So in a way, what you call "simplification" is also a process of "complexification" at the same time, balancing everything out. Also, not all types of meaning are necessary at a given time at language is spoken. More and more, English has lost gender cases, and now even natural gendered nouns are seeing a decrease in usage (such as actress or policeman). We seemed to have decided this is not an important meaning to communicate regularly. However, we are seeing greater complexity in written grammars, especially for digital communication (such as the new usages of exclamation points or periods in texting, which can signify a "smile" or curtness); this is not something Old English felt necessary to communicate in writing this way.

First, some basic linguistics. Grammar is the rule set for a language. There are many components of grammar in languages, and each can be employed to convey meaning. I'll give a partial list of the most important here for our examples, though there are many ways that languages can encode meaning. Phonemes comprise the sounds present in a language. Morphemes include things you stick on words or alterations to words that change meaning, such as case endings, declensions, or plurals. Syntax is our word order. Lexicons are the words present in a language.

I will stick to English for examples, since I, and probably most of you, are most comfortable working with English. You say that Old English and Middle English are more complex than Modern English, but is this really borne out? We have to caveat everything by acknowledging that there are many Englishes, there is no one Old English to compare to a singular Modern English. Sure, Old English had a more complex noun gender system than Modern English. But I can still communicate gender today if I want to, just using different morphemes or lexical choices. So I can talk about a female tree, or a bro pad. But Old English also had far fewer tenses than we do today, with only two marked tenses (past and present), whereas today we have four primary tenses (past, present, future, and future-in-past), along with 16 other verb constructions to indicate nuances of tense.

I don't want to list a lot of other comparisons between types of Englishes, but I will add a few more recent complexities that English has seen introduced that also undermine the idea that simplification is the normal path of languages. I believe a new future tense marker in informal English has appeared in the last few decades in "Ima" or "Immuna." We can see this in "Ima go back to school tomorrow" or "Immuna get a new dog." This form derives from the older phrasing "I am going to" but it has been reduced (or simplified) by elision of phonemes down to "Ima" and is being used to mark a near future tense.

Another major new complexity is emojis and emoticons (and lots of other internet speak). Emojis, as much as Reddit hates them, have taken on a major role in English and many languages. They are used as non-spoken (though some have picked up names) communicators, which is particularly useful on a global platform. Emojis have been named the OED Word of the year in 2015 (😂). They have also been used as evidence in several court cases, such as whether ":-P" communicates that issued threats were made in jest.

Finally, we can see the bevy of new words constantly being introduced into English, not just in the recent past. Since Old English, thousands upon thousands of words have entered the language from many sources, especially Old Norse, Norman French, Latin, Greek, several North American languages, and a lot of the languages encountered during the expansion of the British Empire. Other new words are constantly being created using English grammatical rules, such as manspreading, me-too'd, vape, or yeet. New words come with their own complexities of usage and manipulation. "I yote the pencil" (playing on what you might call a complexity of strong verbs that has become less common over time).

If you are interested in language change, an accessible book is The Unfolding of Language by Guy Deutscher or The Power of Babel by John McWhorter. Or, for English in particular, McWhorter's Words on the Move: Why English Won't - and Can't - Sit Still (Like, Literally) is a good look at current trends in language change.

Oh. This is one I can contribute a little bit on.

Ancient Chinese texts are really hard. Like really super ridiculously crazy hard.

It's so hard that I've got a tattoo on my leg in something that is intentionally gibberish (text from XU Bing's "Book from the Sky") and Chinese people regularly try to read it, fail to understand a single character, and simply assume that it's words they don't know rather than "it's gibberish" because complicated words they don't know makes more sense.

This is not because ancient Chinese was so much more complicated than modern Chinese, it's because the guys who were writing stuff were stuffy jerks, proud of their accomplishments, and determined—from a social status point of view—to exclude everyone else.

Not that they necessarily would have put that in words, but being educated enough to read (and even moreso to write) in ancient China was a big fucking deal. So much so that just doing well enough to take the imperial exams (not even to pass them) could get work for people (maybe not the work they wanted but still work).

Like anything that human society anywhere has gotten it's hands on and made into a marker of social status, you've got to keep that status. So, instead of writing the way people talk, they use clever puns and choose words in ways that look aesthetically pleasing together in order to show off to other equally high status people that they have the necessary luxury of time to spend in pursuits that aren't actively making them money.

(I wanted to use The Lion Eating Poet in the Stone Den as the text for my tattoo. However, despite the fascinating cultural context surrounding this poem, the written form is objectively ugly even to people who can't read Chinese.)

If your only exposure to ancient Chinese was the clever bits being shared back and forth by the scholar class (and not the Annals of [Pick Any Place Name in China]) you'd be perfectly right in saying "that's crazy difficult" but it's not so much because ancient Chinese as a whole was difficult as it is that the specific subset of written Chinese that mostly survived long enough to be ancient would have been difficult when it was new.