when there /weren't/ any such guidelines. And it's not bad advice now, since the 5e guidelines are a little unwieldy to begin with. But it is kinda just giv'n up, in a way, and it could very easily end up with a campaign pacing that, yeah, that party can handle, and handle well, but which does also leave you with distorted class balance. In turn, of course, players can adapt, too, and stop playing classes that don't work well at that pacing....which then moves your target for 'what the party can handle...' and so it goes...