Certainly it isn't a issue after they cats no longer live there and it's been cleaned to a reasonable standard. If it knocks 20 grand off the price of the house it's worth spending 2k to have everything deep cleaned.

Cat pee permanently stains flooring and is also extremely hard to get the smell out. 2k will not be nearly enough if there’s extensive cat damage. Wood floors turn black with it and must be replaced.

Yes, even after extensive renovations cat pee smell can persist and some people are bothered by that smell. Im one of those people but I do like cats and wouldn’t mind having cats if they wondered around the neighborhood rather than be inside only.

Killing small animals and birds.

There are billions of old and ill small animals and birds each year who would normally be taken care by various predators. Around humans pretty much only cats can do that important and necessary job. The popular Smithsonian study that people like to cite which states billions of killed by cats somehow fails to mention what share of killed are ill/old vs healthy. There is no surprise though in such a glaring deficiency of that study once one learns that that Smithsonian team has a researcher convicted for animal, specifically toward cats, torture.

Another aspect - rats, a human civilization companion, raid nests for eggs thus decimating birds population around humans. By controlling rats cats help to maintain birds population.

This is not true, see my reply to your sibling comment for a more detailed answer. Toxoplasmosis is highly correlated with severe mental disorders like schizophrenia.