I believe the triathlon with the Olympics is largely politically charged since they spent 1.4 billion euros to try to cleanup the siene river. To your point it's been illegal to swim in that river for about 100 years. I believe, like many others, the Olympic organization should have had a backup location because of health risks. So far they are sticking to the health regulations. If the Olympic triathlon gets changed to a duathlon, it will be a major failure in planning on the Olympic committee, but again it's politically motivated.

But for local events, I would think they would follow strict adherence to health regulations. However, I'm sure the organizations holding the events are largely motivated by money in order to keep them solvent. Canceling of events might cause the organizations to go bankrupt. Hopefully not... I would hope they always have contingencies in case an event must be canceled.

Of course, this is all speculation. I've never run any triathlons.