Yes, exactly! But… the one-in-one out rule is so very hard to enforce.
It would be easy if I intended to continue playing guitar, due to the expense of guitars I consider worth buying and my lack of storage space. You are correct in that it is much harder to enforce where whistles are concerned. If I felt I couldn’t enforce it, I’d just wind up spending $20 on another drumstick bag!
I made a similar experience – I also used to play guitar and I have just two, one electric, one acoustic. Never felt the need to get another one. But not so with whistles. However, considering the price difference, I could probably get some more whistles 
.
I know that, given my lack of cat-resistant storage options, the third guitar would have meant either the first or second would have to go. And as I contemplate a considerable downsizing…it’s just not practical to keep playing guitar. Building a collection of whistles, on the other hand, requires much less investment and storage space. It’s no wonder WhOAD is so prevalent!
Returning somewhat to topic, I put the Little Black D through its paces a few moments ago. It sounded particularly atrocious, so I finally made it tunable. Much to my surprise, the inside of the head was free of flash and other molding flaws, but a glob of unidentifiable crud had collected on the body at the inside of the head end. After removing that crud and experimenting with new positions for the head, it sounded a little bit better. I see a whistle tweaking in my future.