There are valid arguments both for and against but the one most commonly used, that they decimate bird populations, is heavily contested and not actually found in evidence.

Just because cats kill birds doesn’t mean that those birds would have otherwise made it to mating season. You have to actually check that, and most papers into the subject never bother to do that part, mostly because it’s actually quite hard.

Actually one of the best arguments against letting your cat outside is not to protect birds, it’s to protect insects. The defleeing liquid the cats are often given gets into the water and as a devastating effect on aquatic insect life. You shouldn’t be giving your cats flea treatments unless they actually have fleas and then you keep leave them inside for a few days.