If you have something of good enough value, you should be able to get something cash in return.

It could be that you're not creating enough value? Sure, there's some. But true product market fit is when they beg to throw money at you.

It could be that you're using the wrong revenue model. Firefox, Wikipedia, Facebook, Google all make a lot of wealth/value, and they're also pretty rich. But nobody directly pays for the service.

To quote Paul Graham:

"If you can just avoid dying, you get rich. That sounds like a joke, but it's actually a pretty good description of what happens in a typical startup. It certainly describes what happened in Viaweb. We avoided dying till we got rich."

There is one reason to not shut down: you have a lot of data on what didn't go right, and you could use that data to see where else to go.

The only real reason to shut down is either you've run out of money or willpower. Even in that case, you could still sell off the site to competitors as the domain name and newsletter itself has good marketing value.