So, I got a response from Blogger. It's pretty much what you'd expect:

Hi there,

The optional split-screen blogging interface preferred by some users has
been replaced. Instead, one improved and streamlined interface is
available to all Blogger users. Our decision to make this change was a
response to user feedback and testing.

The new editing environment is a better workflow for the majority of our
users. Additionally, supporting one default interface as well as one
optional interface to Blogger was a strain on resources not to mention
potentially confusing.

That being said, we have noted so far the major features of the old split
screen blogging environment that people miss most. Fans of the two-pane
layout were fond of the context it lent to the workflow; you could read
your old posts while composing a new one. They also liked the calendar
interface for browsing older posts.

Please keep in mind that this is the first iteration of the new interface.
We continue to gather feedback and monitor response to this upgrade.
Innovation and the implementation of new features is ongoing. Helping you
create and browse your posts is a high priority and we will continue to
enhance the Blogger editing environment for a good experience.

Sincerely,
Blogger Support

They start of by telling me that the split-screen interface preferred by "some" has been replaced. Well, no shit. That's what I'm bent about.

I don't see how they consider the new interface "improved" and "streamlined." I think I know what it is, though: laziness. As one commenter pointed out, many people out there have never been able to take advantage of the split-screen interface. This is a browser-dependent thing. Sometimes when using other browsers than my usual one, I would get a shitty interface that's much like the one we're all stuck with now.

So, instead of working to get the decent interface going for everyone, they canned it and gave everyone the shitty one. Okay, look, I can understand this in a lot of ways. With all the crap you have to do to get stuff working cross-platform, it has to be a huge pain in the ass. But isn't Blogger owned by fucking Google? Surely they have a few extra dollars they can throw at either working towards getting the good interface working for all or to help maintain two different interfaces.

I sincerely hope that this is just a first iteration and that things improve, but I'm not holding my breath. It sounds like the good ol' days with the split-screen interface are gone for good. The only hope that we have is for more people to bang their monkeys and tell Blogger how much the "improved" interface sucks. Actually, probably the only way to get their attention would be for Blogger users to start jumping ship en masse.