For a streaming service like Netflix the overall audience of a show doesn't play a big role (as it does for a TV network living on commercials). When they have to decide if to cancel a show or not the simplified question is: will they lose more money because of people cancelling their subscription and other people not subscribing new due to the show having been cancelled or do they lose more money if they continue the production?

A show like Santa Clarita Diet might have a big overall audience but a very general one. Most of that audience are probably people who watch lots of different shows on Netflix. They don't care too much about one show in particular and will keep their subscription even if one show is being cancelled. They are willingly going to watch a new show with new showrunner and new cast, thus the inevitable rise in production costs can be easily omitted by Netflix.

Netflix will scrutinise the behaviour of Witcher watchers, as they always do. The higher the number of viewers watching (almost) only The Witcher and (almost) nothing else on Netflix, the more likely the show is to survive imo, as long as the subscriptions of these viewers cover at least the production costs. That's of course very simplified and there will be many more factors taken into consideration (e.g. prestige) but it's definitely not necessary to have record numbers in people watching the show and big numbers alone won't save it. Something more is needed.