Isn't a toast just the tail of a message log that's visible for a few seconds after a new message is added? I don't always want the entire log to be occupying space on my screen, but I do want to see when new items get added.

The problem is that everyone implements toasts, and no one implements the message log, even though it's the latter that's more important to have.

Exactly - a toast is, at best, a nice add-on to a message log. Although I maintain a well implemented message log already serves every single purpose toasts do, better.