Hiragana or Katakana are not Chinese characters. They don’t follow the principles of how Chinese characters were developed(六书), nor did they follow the structures.

At best they are inspired by Chinese characters. No one in China or Japan will classify kanas as Japanese made Chinese characters, which by itself, meant something different entirely, like 畑/辻/雫

イ developed from 伊, ク from 久, め from 女 and れ from 礼 in much the same way that 余 developed from 餘, 区 from 區, 汉 from 漢 and 礼 from 禮.

Wantonly altering character shapes to make them easier to write quickly may not be a blessed way to create new characters, but it has tradition. It's not like the Oracle Bone Script was passed unchanged from generation to generation and only the moderns dared lay hand to it. Lazy scribes have existed for as long as there have been scribes.

That hiragana and katakana are now treated as a completely separate category is more down to them being used very differently than how they were derived.

«Wantonly» is perhaps not the most accurate term at least in the context of hiragana. It evolved as a writing script for women and used exclusively by women and by women only due to the Japanese women having been denied education in Japan at the time and having been considered too inferior to learn kanji. The men scoffed at hiragana for many centuries as something relegated to the «crazy» women writing love letters and expressing «lowly» emotions in a correspondence with each other. The Japanese men would only write using kanji. Eastern Asian mysoginy has a pretty rich history that has even found its reflection in a writing system.

Fair, I think the difference here is that by simplifying Chinese characters into Kana, it also removes the semantics from the character completely, but that is also not completely unheard for Chinese either (like 的, for example).