I don't find interleaving natural. It would be natural if we were using old school hypertext and reading from anchor in the original text to response in the new text. But as it is, I have to skim a whole email again to find the small portion that changed. If the reply is terse enough, I have to reread the large original to get enough context to understand what they are saying.

If the large portion is snipped, you can't read the whole thread in the final message(s, for forked threads), but have to go spelunking through multiple messages.

Also, on some mail readers it is hard to tell the difference between responses and text indented for visibility, like quoting or code or equations. Replies to replies get really messy.

I also don't like scrolling.

I got used to reverse chronological reading with blogs (the train has almost sailed on them, huh).

What top posting forces you to do is to form one coherent complete thought (if you plan to be understood).