So, if the utility does not outweigh the cost, simply don't mine it. Leave it to be mined by people who want to test. Mining a worthless coin with the hope of selling it for a price makes no sense. If no block has been found in 20 minutes, then the difficulty resets back to minimum, so cost is ~0.

The last reset was 10 years ago, because it's worked fine by now. If we start setting up "testnet marketplaces", I'm sure people will either abandon testnet and go to signet, or just create testnet 4.