I have my own knowledgebase in my own format and TBH I'd say it's one of the most valuable things I own. I don't think the second brain "marketing grumpf" is false - once you get rolling, you have a personal treasure trove of knowledge without having to dedicate every piece of info to memory.

The value of whatever style of note-taking suits your work and personal style has been well understood for centuries. All the productivity piffle has added nothing to it, though it has earned a few bucks from book sales, youtube ads, and merch.

I had doubts about the second brain stuff as well, but I am seeing that with the connections I make to my notes using links/backlinks tags while hanging out in the graph view allows me to see connections between notes and helps me with idea generation, so I can see why people call it a second brain. It helps you think about writing in a different way. That has been my experience though.

It's the messianic fake novelty and loathsome invented marketing terminology that I'm objecting to. You can unfussily make notes, using your own emergent organisation exactly like people have done for centuries at least. Tech can add searching and organisational niceties, and it makes sense to use them. Obsidian is good at this (I use it).

Or, alternatively, you can do exactly the same thing and get terribly zealous about it as a grift, or a way to mask the fact that your notes are all about notetaking systems and just add nothing to the sum of human (or even your own) capability.

I'm not arguing against the activity of taking notes, nor working with different representations and arrangements of them to discover what emerges, nor against note-taking software, just all the timewasting blather that goes with it. "Second brain" (probably TM'd somewhere) is by the way only a grey businessy echo of Andy Clark's (much more interesting) 'extended mind' theme.