It is, too.
Though not often said these days.
I’m trying to think of an instance of it that isn’t in an old movie or song, or play, for example.
“In the sweet by and by…” a song, I think.
And it’s one of those phrases that if you elide it a little bit, it starts to sound very odd. Nor would it be entirely clear what it meant if you didn’t speak English, I imagine. Or just never heard this expression.
Hereafter means essentially the same thing but “by and by” has a sense of lackadaisical ease to it that “hereafter” lacks. And hereafter breaks down to the elements it is when you look at the words. It’s – here – after.
But “by and by” – one of these “by”s on its own would be meaningless – and it doesn’t break down to my sense at all.
But it is easily said.