Citius, altius, antibioticus. On a clammy Wednesday morning in the 8th arrondissement Paris 2024 staged what is, quietly but in plain sight, the keynote event of France’s entire Olympic project.

The women’s and men’s triathlon competitions were held in a loop, returning each time to the startlingly beautiful Pont Alexandre III – also known as the bridge in that Sopranos episode – and taking in Champs Élysée, Grand Palais and the western vista of the Seine.

In the event it was a genuine triumph, a race across water, cycleway and Parisian cobbles that will have left Emmanuel Macron punching the air in his Élysée office, for reasons that go far beyond the optics of a wonderfully stirring French gold for Cassandre Beaugrand in the women’s race.

First, just because it happened at all. By early afternoon there were already conspiracy whispers of executive action taken to get this thing on, eyebrows raised at how, after days of prohibitive pollution readings, after talk of cancellation and the political disaster of being tagged for ever as the sewerage Games, the level of pathogens dropped just enough to allow the event to proceed. Sometimes, $1.5bn into your de-pollution project, you just get lucky.

Second, this was an aesthetic triumph, an impossibly beautiful and luminous event, the kind of moment where Paris gets to flex its shoulders and it becomes necessary to marvel at the splendour of what humans have managed to do here.

Third, this was a sporting triumph for the Games. Three hours after Beaugrand had won the women’s event, Brockley’s Alex Yee did something equally astonishing, mustering up a sprint finish at the end of a 10km run over ankle-ripping cobbles, and collapsing at the tape to take gold for Team GB. Close up, witnessing the strain on faces and bodies, this was surely one of the greatest feats of sporting endurance you’re ever likely to see, and in its own way the event of the Games.

Finally, and most significantly, this was a triumph of optics. What happened here exactly? One hundred and fifty people cycling in spandex? The most brutal endurance event seen in this city since Friday’s opening ceremony?

Zoom out a bit, and what Paris staged on Wednesday morning was basically a centrally approved extinction rebellion happening. In plain sight the Games, the athletes, the progressive mayor Anne Hidalgo and an unwitting International Olympic Committee gave us direct urban action. Close the roads. Grab the eyes of the world. Cycle, swim, reclaim the streets. End up slumped on the kerb being carried away by men in high-vis vests.

It is one of the undiscussed benefits of Paris 2024 that closing roads, ramping up cycle lanes and opening urban walkways has created a really lovely version of this city. The triathlon turned that into a sporting spectacle, a vision of city-friendly carbon-light travel, as enacted by cartoon champions in trainers and two-piece swimming suits.

There was always an amusingly blinkered note to the pre-Olympic gloating at the state of the Seine. For the British media this was basically a classic opportunity to titter at the French, presented as just another continental toilets story, a throwback to Bidet Panic, to the long-drop latrine horror talk of the 1980s. Trying to clean a river now. What will these freaks do next?

In reality waterways in every country are full of sewage, not least in Britain where the lack of care by privatised water companies, the surge in sewage overflow, the unswimmable state of lakes and rivers, is a scandal of monstrous proportions. Somehow the most basic of all human needs, the element we are basically made of, is being shaved to the bone for the benefit of shareholders.

Does cleaning the Seine still sound like such a folly in this light? We hear so much scoffing at the €1.5bn spent. But what do we think this is really worth? When does a clean river become too expensive? These big events are always soundtracked by drivel about legacy and regeneration, words that are used almost exclusively to sell you a scheme. But cleaning the Seine is undoubtedly the best thing Paris 2024 has brought to Paris. What better legacy could there be than don’t throw shit in the river.

The challenge now is to maintain that beyond the current two-week TV show. This is where the optics come in. The insistence on actually hosting swimming events in the Seine may have been deeply pig-headed. It may still turn out to be a terrible idea. Team GB’s athletes have already had their hepatitis jabs and will now complete a course of antibiotics. Who knows what the effects might be of all that tarnished river water combined with the cuts and wounds of assorted crashes, wipeouts and knee slides on the morning cobbles.

But the message was in the spectacle. At times, watching the switchover between disciplines, people in swimsuits leaping on and off bikes then pounding away frantically on the cobbles, was like witnessing the world’s worst-ever commute, or a real-life version of that dream where you’re doing all this in the nude en route to giving a full board presentation about project Zeus. What is the point of this deeply stirring display of human will, other than to say, well, some things are just worth doing.

The Seine is, after all, why Paris exists in the first place. The Romans made it a trade route. The Vikings pillaged it. France’s own belle époque expanded along its banks. The same river water the athletes were glugging down here also contains ashes of Jeanne d’Arc and the debris of the second world war.

And like every major old-world city this place is built on a labyrinth of inherited tunnels. With Paris the key task in this Olympic cycle has been to build a giant rainwater tank. The existing Napoleonic infrastructure takes rain and foul water down the same pipes. Hence the toxic overflow when the rain gets too heavy.

Climate change hardly helps. Talking of which, Paris plans to open 26 swimming pools along the Seine, in part as a basic response to the question of how to live in this city as summer temperatures rise. Public bathing. Fish stocks and river foliage. This is the idea. It may or may not come to completion. Hidalgo has already been under pressure over her anti-car reforms. But does it really sound so bad? Paris is basically flagging a problem shared by every city in the world.

And so for one morning at least the triathlon – a swim, a bike ride and a run – felt like the centre of the Games. It was above all beautiful, from the silver-grey light on the Seine first thing, to the sun through the clouds as the runners emerged dripping river weed and slurry, pounding off on a kind of amphibious parkour. By the end it seemed clear that everyone involved in this event deserves some kind of medal, not to mention a course of antibiotics and a typhoid shot.

Paris 2024 delivered its message here. The challenge now is to hold on to that image. And for Macron, to fulfil his own promise of swimming in the newly cleansed Seine, an ambition Jacques Chirac also shared but never lived to see out. No time like the present, M President. L’effluente, c’est moi.