Are people mad for visiting countries abroad during this time?

If I were to wait until travel abroad was "completely safe" I might as well burn my passport right now.

My husband and I are planning to visit a major European city next month or early Sept. We reckon it'll be worth the risks just to be able to wander around in peace the way you never can in normal circumstances; also flights and hotels are (naturally) cheap at the moment.

If we have to quarantine when we get home it won't be a problem as we're retired and have managed lockdown very comfortably.

For people in our situation there is no hugely compelling reason NOT to travel.

PS We're British.

I would agree that its very unwise, and I wouldn't even recommend travel within one's own country, either, until the pandemic dies down, unless you are willing to be the one to die. 

I think you're mad for stating "until a vaccine comes next year".

I hope you are correct but I wouldn't count on it.

"until a vaccine comes next year"    You are counting your chickens before they have hatched.

IMO yes

But if they are from the US theres only a few countries  letting them in anyway

It's risky to travel right now and there are lots of restrictions on foreign travel. For example, many countries will not allow tourists from the US due to the infection rates in the US. It's not something I would consider right now.

It's probably not the best time to go just for a regular visit right now. But in the case of saying goodbye to a dying relative or something, many people might be willing to risk it. 

I don't think people NEED to go on vacations right now and I don't think that's a good choice.  But I also don't think it's my place to question why people might be visiting family or otherwise traveling right now. 

I have a good friend whose parents live overseas and her mother is ill; she's devastated that she can't go visit them right now as she normally does in the summer, and I completely understand why she wants to and would not think twice about it if she left. 

Unless it's unavoidable - there's no good reason to do so.