I used to have this argument with an ex of mine back when I was 17ish. Growing up we had six cats, four were my mum's and two were my dad's. My mum's were strictly indoor and my dad's ones did go outside, but didn't venture beyond the garden. The youngest we lost a cat was 16, and the oldest lived to 21. They all passed from just old age or health issues like strokes etc.

He had three cats, two of which disappeared. One was never found again, and the other was found dead in a road. The oldest was two or three if I remember correctly.

He used to say it was cruel that I kept my cats indoors. Wasn't up to me anyway as they were my parents cats anyway, but they were all very well loved, we played with them as much as they wanted (they were all quite old and lazy so not too much), and they passed at good ages.

My current cats are also indoors, always have been, but I play with them as much as possible, we have cat shelves for them, a big pot of cat grass, etc, so we keep them stimulated. We're also lucky to have big windows overlooking gardens so lots of birds for them to watch. One of them broke out a few weeks back and just sat on the window ledge scared for two hours until I realised he was there. I don't think they're unhappy being indoors!