I can't speak about AOL because I never used it, but I imagine the parent is talking about how apps like Outlook killed email conversation threads. [Edit: apparently that's not what parent was talking about, but I'm happy to take the opportunity to rant about a pet gripe anyway]. Far from simplifying email, they took a fairly straight forward system and complicated it to the point that nobody understood how to use it any more.

Specifically, they made conversations almost impossible to have by making top posting of replies the default, coercing the user into a behavior model that was contrary to the best practices that had been established by users of email to that point. The predictable result of this is that people stopped bottom posting and conversation threads were made practically impossible.

I wouldn't be surprised if this is why forums took over from mailing lists. I also wouldn't be surprised if this is why Gmail became so popular, because it fixed a problem that should never have existed: it made conversations comprehensible again.

So while I wouldn't blame newbies for breaking email, there is definitely a strong correlation, and it's because of the inadequate tools they were provided with in those early days.