Basically all spinners on the web work by sending a request which causes the spinner to show up, waiting for a response which causes the spinner to be removed. Many of them don't handle errors which causes them to keep spinning.

A static loading message is exactly the same, while the message is on the screen the request is still pending and while it is gone it is not. The only case you are not catching is where the browser locks up and becomes unable to render the spinning which is not that common or useful.

Maybe but it's the common implementation of a spinner.

I can't remember the last time I saw a spinner that actually showed some kind of variable behavior. If you have that, it's more likely you are being shown a progress bar.