Frankly, I don't know the answer. Here is some common knowledge for Chinese you might be interested in, and possibly remotely related to this issue. But I'm by no means an expert, so please take a grain of salt.

> why did it then evolve to the more complex form where classifiers are mandatory?

Simple is not necessarily easy. The ancient Chinese is compact but not easy to learn.

The Chinese, especially the literary/classical Chinese[0] (as opposed to vernacular Chinese[1]) was very scholar oriented. It's pretty bizarre to the rest of the world because historically the government and scholars actually have strong influences and even controls over the language itself.

There were even many times like 'Hey, you know what? We believe the language is not good enough, and we're going to fix it!', and officers and scholars were assigned to refactor the language. The recent one leads to the traditional and the simplified Chinese diverse from each other. According to the legend, Cangjie[2] invented the Chinese characters, and it more or less becomes traditions, or at least the legend could help the government claims to refactor the language.

The ancients scholars tried to make the language compact and precise. However, the cost is the language is pretty cryptic. If you know a lot of idioms (usually 4 syllables), you would find they don't really make any sense unless you know the whole historical events/stories behind them.

Scholars from Korea, Japan, and Vietnam knew and love Chinese. Many of them tried to evangelize the language to their own people. But they never made it to the average people, though the were many strong influences till the modern days.

> Typically languages tend to evolve to be simpler.

Also, interestingly, the redundancy of the Chinese language actually largely increased over time. My personal take is it tends to be easier rather than simpler.

Some linguistics split Chinese into 3 periods: the old Chinese[3], the middle Chinese[4], and the old Mandarin[5] (the translation is 近代汉语 - is 'the near modern Chinese' if we translate it directly back to English). The middle Chinese period starts from the Northern and Southern dynasties (start from 420) to the Tang dynasty (end at 907).

In the beginning, during the old Chinese period, Chinese people tend to create new characters/syllables over new words (which might combined characters). And that's why the Chinese have so many characters nowadays. But the trend is not sustainable. To be frank, Chinese people may remember thousands of Characters, but they might not remember hundreds of thousands if the trends persist. Also from the syllables perspective, there would be not enough combinations.

So the middle Chinese period, the trend stopped and people started to combine two characters/syllables together for a word, this is especially common in vernacular Chinese. The vernacular Chinese during the middle Chinese period is actually pretty close to the old and modern Mandarin.

Also, Chinese people more or less having OCD on making characters/syllables in the same length so they would sound perfect, especially look perfect (Yes!) - poems usually have 5/7/9 syllables for each sentence. Words and idioms usually consist of 2 or 4 syllables. If a lot of words are made with 2 syllables, they rather make them all 2 syllables.

So the original common words would have a lot of redundancy, for example, 房(house)屋(room), actually refers to 房(house).

Back to the original question, my personal guess is there was no standard way of classify things, once classifier was more and more useful (because there were going to be more 'cup or bucket' issue as the society develops), the OCD Chinese people want to add fallback so it can make the expression balanced just like how they squeeze sentence 5/7/9 and words to 2/4 syllables. So it would be like 'two pairs of gold bowls, and forty grasps of golden keys'. But that's only my guess :)

[0] https://en.wikipedia.org/wiki/Classical_Chinese
[1] https://en.wikipedia.org/wiki/Written_vernacular_Chinese
[2] https://en.wikipedia.org/wiki/Cangjie
[3] https://en.wikipedia.org/wiki/Old_Chinese
[4] https://en.wikipedia.org/wiki/Middle_Chinese
[5] https://en.wikipedia.org/wiki/Old_Mandarin