Long ago, every email would be read on its own, so writers would include a bit of the previous message to remind people what they were talking about:

On Fri, Apr 10 Pat wrote:
> I miss you a lot!

I'll miss you too.

> Are you still planning to visit in February?

I hope so!

Some people would be lazy, and just include the whole earlier message, without any trimming. This was widely viewed as rude, and there were many "netiquette" guides that reminded people that trimming was polite consideration of your reader.

As time went on, however, mail clients adapted. They learned how to:

• Thread messages. Messages didn't have to stand on their own anymore because the context was right there for the reader.

• Collapse large quoted sections. Including the whole parent message no longer caused trouble.

We have mostly standardized on a new system, where mail clients default to including the full earlier message, and where they hide this quoted section by default. This means that when someone is added to a thread in progress, they can look back at earlier sections if they need to. If someone would like to reply inline they still can, but people only do it when needed.

Mobile has also contributed, where composing inline replies without a proper keyboard is a lot of work, and people often just want to send a quick response.

I finally switched over in 2013 and I was already a bit of a hold-out. Looking through the last 25 replies from other people that I have in my personal email, 24 were top posted and one was bottom posted without any trimming.

I'm on a non-technical list where some users are trying to convince everyone else to always bottom-post and trim, and it's a mess. People don't realize they're top-posting and don't know what they should be doing differently. The most common response has been for people to send each reply to the list as a new message composed from scratch with no quoting at all, which breaks threading and is a pain to read. I think at this point, outside of a few places where there's a thriving trimmed-replies-only culture, we should admit that top-posting has won and adapt any remaining workflows to accept it.