The mice were getting to be a problem, btw, because they were burrowing between the walls of my house and making noise at night, and doing god knows what damage in there.
My wife was once on the porch with him. She was gardening (lots of potted plants, etc). He was laying there in the sun and snapped up to grab a hummingbird in flight. She grabbed him and he spit it into her hand. The hummingbird flew away a few seconds later. That was when I realized how efficient a predator he was.
EDIT: and he did get into a fight with a raccoon once, that required stitches. So, living inside will definitely increase the health and length of life.
Also: https://en.wikipedia.org/wiki/I_Know_Why_the_Caged_Bird_Sing...
Seems unlikely. Citation needed.
'A pet that spends more time outdoors may live on average 2 to 3 years less than a cat that lives exclusively indoors.'
That indoor cats live longer seems reasonably well attested. However here's some anecdata: sum of my parents' 8 indoor/outdoor cats + my 3 indoor/outdoor cats: none have lived less than 13 years (two made it to 18). My parents two indoor-only cats both died before reaching 13. I have assumed the cats that go outside survived due to the low number of vehicles and predators they had to deal with.
There are other reasons to keep cats inside, as is the subject of the article. The problem is much worse in Australia where birds and small marsupials nest on the ground.
In the country, I could imagine letting cats outside -- though only after getting them neutered or spayed -- but in the city or suburbs, where I have lived, keeping them inside makes more sense.