Not having ANY tests means tons of manual testing is needed every time you modify code, which will rapidly consume more time than writing the tests would.

The manual tests also stop being run, or get reduced to such an extreme that any value from them is going to be low. Testing only happy paths and maybe release specific tests. This is shockingly (to outsiders, not to anyone who's ever been in the industry) common in aerospace and defense systems. There were some aircraft I would not fly on for a few years until I knew our updates had rolled out. Now I'm not connected to that work anymore so I'm back to "ignorance is bliss" mode and try not to think about it.