Can you show me any numbers for this, broken down by type of area? Outdoor cats obese? I've never seen any here in Germany. Ever. Indoor cats: many. Cat's eat prey when they're hungry. And they can't hunt enough to get obese. They get obese when they have access to food all the time. This is an owner-made problem. When neighbours gave our cat food once for a while we told them to kindly stop and explained why and they did.

Attacked? By whom? Cats fight for territory; this is natural.

For every point you bring I can bring several anecdotes that counter it since I grew up in the countryside and with cats. And they are anecdotes -- just as what you said is w/o substance w/o any statistics. But here I go anyway:

Our first cat who lived until 17 years was wounded twice by other cats and once by rats. We had to have her treated only for the rat attack (antibiotics and some stitches).

But that's expected. I had several accidents in my life. Certainly, if I never left my apartment/home/house they could have been avoided. But maybe then I would be depressed or have killed myself already ... unless I had access to window of course, to entertain me ... pardon my sarcasm.

Seven years? I grew up on the countryside, village outside of Hamburg. We always had cats.

After the first one died a neighbour's cat choose my parents as new can openers because the neighbours separated and the woman took the cat and the children and moved to an apartment on the other side of town. The cat just escaped and ran back to his territory. After the third time this happened the woman gave up bringing him back.

He died four years ago at the age of 18.5.

Seven years? No evidence for this here. With all the cats of our friends. One just died after 25 years. And they live in a small town with lots more traffic than my parents.

Same goes for all the cats in the neighborhood of my parents house. I know most of them because they 'visit' as their territories overlap.

Yes, we had friends who took two kittens from us. They lived next to a big road outside the village. After both got ran over by cars they stopped trying to have a cat (instead of opting to have on inside their 350sqm house, mind you). So it's a case by case decision for outdoor cats too.

There are many factors that affect life span of a pet. I would be very careful with correlating indoor/outdoor as you do.

Happiness: definitely. I've never seen an indoor cat w/o issues. They're minor when they have good owners and care but they are always there.

Besides, personally, I'd rather have a shorter, happier life, than a long dreadful one. That's just me and I can choose.
I go out lots, I drink alcohol, I drive a motorbike on freeways and a bicycle in rush hour traffic. All factors that are shortening my life expectancy. I choose so.

Pets can't; no one asks them.
But I would know what side of the answer to bet on if we ever can.

Declawed? I never even heard of this. I do not think this is even a thing in Germany.

I have two friends who are vets. One of them owns two outdoor cats. Never asked them what they think about this.

Imho the choice to keep a cat indoors or outdoors often reflects the character of the owner. People who are more fearful in general, of the world, of their wellbeing, of things that could go wrong, tend to be over protective. Of children, pets, themselves, etc.

As such: it is probably no coincidence to read misconceptions like yours in the age of helicopter parents. Beers!