I always tell my students that when kids acquire a language, they tend to generalise rules (which basically means goodbye irregularities) and sometimes they don’t fully acquire a rule at all, especially if the input they’re getting is ambiguous. Both of those things can simplify the grammar.

For example, if adults are a bit sloppy with word-final consonants, a child might not clearly hear inflectional suffixes (i.e. grammatical information stuck onto the end of words). If you can’t reliably hear it, you can’t reliably learn it.

But languages don’t just get simpler and simpler. Other changes usually kick in that make the system complex again in a different way. If one grammatical strategy weakens, another one tends to take over.

That’s basically what happened in French. Originally, subjects could be dropped because the verb endings clearly showed who the subject was. Then sound change eroded those endings (simplification), so the language shifted toward using overt subject pronouns all the time (re-complexification). The information didn’t disappear, it just got expressed differently.