Gardening helps me see this clearly. I heard it once about farming "if you've got livestock, you also get deadstock" and it happens in the garden too.
I've had a bougainvillea in for a few years now in a pot. It works fine in Singapore - they just water them every day and it's all fine. Last year I lost one about this time, and this year I'm seeing the leaves wilt on the remaining one. I'm pretty sure the roots get a fungus in the cold and that's it.
And when I've had these self seeded passionfruit marigold (tagetes lemonii) they get to a foot tall in a pot and then just die. Second time now. Rootbound perhaps.
Things die, life goes on.