This strange 'weakest birds' idea promotes the notion that cats are doing birds a favour by speeding up their evolution, and you are significantly overegging it.
As I said before, it’s not my idea; this is what the experts in the article said.
Try studying African wildlife and preferably go watch it in action. Read for instance “The Serengeti Lion” by G Schiller, or any studies on the big predators like leopards, cheetahs, hyenas…The predators always go for the smaller, weaker animals - the ones with cancer, the elderly, etc don’t suffer lingering deaths like we do. (As per a safari guide, while we were all looking at a dead kudu, clearly killed by lions - these people spend all day every day in the African bush watching the wildlife)
Study the effects of people putting enough food out for 196 million garden birds (the ones predated by cats) and how those birds, which use feeders are displacing the rarer birds, which don’t!
As a pp has said, blue tits are the
garden birds second most predated by cats and their numbers have increased by 25% since 1966. Blue tits are displacing willow tits, who do not breed as prolifically. It’s the willow tits, who are under threat, not the blue tits. See the BTO articles on bird feeders.
It is likely it is the quarter of a million stray and feral cats, that prey most on birds; and keeping pet cats indoors will do nothing to address that.