The Indoor Cat Initiative has great tips on keeping indoor cats happy. I think you might like it, Lady Jane!
For Cat Owners | Indoor Pet Initiative
It's rarely true that cats can't adapt to indoor life. Most can. The ones that can't are put up for adoption by shelters as working/barn cats. The fact that reputable SPCAs & humane societies rarely do this is proof enough that cats are adaptable to life indoors.
One of our past cats was born at a barn I rode at, sired by the most feral/semi-wild cat I've ever encountered & raised by his barn cat mother 'till he was 6 months old (cat preference is 24, not 12, weeks). He roamed about ever wider stretches of truly wild, desolate territory as, being a he, he was increasingly unwelcome by the resident tom (who wasn't his father; his father was a fly by night, here today-gone tomorrow kind of tom). We took him in cold turkey at a year.
He loved the indoor life. Had to get used to TVs (they really spooked him at first). And he was impressive. Could jump 6' from a standstill & then stare at you like if the ceiling/furniture were higher, that was nothing. Once in a blue moon, he'd go out for a walk on a leash. He didn't lose his barn skills though, & they were put to great use when some mice imprudently decided to move in. He also didn't start to 'play' with them. Their deaths were swift & ASAP. All these people saying their cats 'need' to hunt, because, see, there they are 'playing' with an innocent field mouse or fledgling are liars/delusional. A real barn cat kills. Period. It's not a game to them.
So, I don't believe 99% of cats can't adapt to indoor life. Here we come to the real reason they're shuffled outdoors: human convenience. Some people are incurably lazy, at least as far as animals go, & don't want to clean a litter box. Cats are such easy pets. If you eliminate the litter box, it's almost like having an adult room mate, rather than a dependent animal. They will cite all sorts of 'needs' & 'instincts'. At the end of the day, like dogs but unlike all the others, cats chose to be domesticated. It's not a crime to not want to clean a litter box. But it is if you then get a cat & impose it on the resident wildlife.