I find it useful to have everything in a single buffer, it makes searching easier. Otherwise I'd have to start issuing 'grep' commands and worry about which file is which, and get clever about regular expressions when all I want is instant, friction-free access with a handful of keystrokes. Opening a file that big doesn't take long on modern hardware.

Honestly, there's not really anything to "maintain". Just a text file and some timestamps. No magic at all. Simple as dirt. That's the magic :-)

Searching visually? Yes, 100k would be impossible. But then, even one year's worth would be impossible. So you're surely using your editor's search function, and that's definitely fast enough to not care whether you've got several lifetimes' notes.

A do see a lot of people who are making date stamped files, but I can't understand why. Do you want to switch between using C-s vs dropping to command-line for grep?