Heatstroke is seriously bad news .I got it pretty badly once.....started making poor gear changes, getting irked at other drivers........just about got to a hotel . Straight into a lukewarm bath to get my temp down and promptly lost control of my bowels . Good that you recognised something was wrong and did something about it . It can definitely kill you no doubt about that

First time I ever did a long European ride in the summer I suffered the same. But I didn't know what it was and spent three days lying in a tent trying to recover. Even after that the next two days of riding are a blank in my memory. So yes, very dangerous. Fortunately there's enough info about recognising the symptoms and what to do that I'm unlikely to suffer a repeat but there will be others for whom the issue doesn't feature in their planning and may not recognise what's happening to them.

Even now with global warming the UK rarely gets days hot enough to cause serious dehydration or heat exhaustion on a bike but head south and it can quickly become a problem. The prospect of riding wearing an 'armoured shell' of protective clothing more suited to a UK winter in over 40C heat and significant humidity is not one I would want to embrace. Dehydration in particular can creep up on you and you don't start to notice it until you're well down the path. By then you'll need more than a mouthful of water to put things right. Every single long distance running race I've done I've ended up dehydrated despite there usually being ample water stops en route. It's hard to recognise the symptoms despite knowing it's going to happen and easy to ignore until some other consequential issue slows you down.