It's really unrealistic that all current devs would stop contributing to the code. Covid would need to have like 99.9% lethality to kill all Bitcoin devs, so we'd have bigger problems than Bitcoin development if that happened.

Over the course of Bitcoin's history many people stopped contributing, like Mike Hearn or Gavin Andersen, and as you can see, it's not a problem. In the highly unlikely event of mass quit, the development would slow down for some time, but eventually it will get back on track.