A student goes to FOSDEM , catches COVID-19 without any symptoms, and then on the way back home ends up interacting with a bunch of elderly family members, infecting them and possibly hospitalising them in the process. That's just one scenario, there are hundreds of other scenarios that will also occur.
Why take the risk?
Most people have some kind of underlying health condition (more than two-thirds of the western world are overweight or obese -- that's already one co-morbidity), so it's not surprising that most people who die from CoVID have other health conditions. And this is ignoring the more obvious problem that even otherwise perfectly healthy people can infect family and friends.
This would be fair enough if it were the case that only those choosing to attend the event are taking the risk. Most would-be attendees presumably live in societies where they interact with others, and presumably would need to interact with others travelling too and from the event.
For example, anyone who decides they want to risk infection should have to register with the government and they will be provided with an arm band that must be on display in public areas so other citizens can avoid them. Private premises will have the legal right to deny access to anyone with the arm band.
It might be a fun experiment in natural selection taking is course. All the "callous" types can go to FOSDEM and and see how bad of an infection of COVID-19 they -- or their relatives -- will get, and if they are part of the 0.3% (if that is accurate).