It's both, but more so for the current passengers. While not thorough, they do clean the planes between flights, about as well as they clean movie theatres between viewings, maybe a bit better.

With previous occupants, the damage is done, nothing can change the fact that they might have contaminated an area that wasn't properly cleaned, but limiting additional contamination is the only mitigation available for this lady, besides traveling in an N95, wearing gloves and other PPE, which may be a better option than buying up all the offending material.

I can sympathize with her plight, but I understand that not everyone will; and asking people to be cautious of their surface contamination... Well, you might as well ask them to will themselves through the air to their destination unaided by any machine. IMO, most average people don't understand surface contamination, and by proxy, won't participate in such restrictions. It's not something that most people need to, or choose to think about.

I don't begrudge the person for doing what they did, they have a medical issue that nobody took seriously enough. I get it, but I'm sure more than a few passengers were salty about not being able to get peanuts... No pun in tended.

There's also the contamination of the flight assistants, someone eating peanuts and having a beverage, contaminates the beverage container with peanut oil, which is collected by the attendant, whose hands are now contaminated, even if they ate no peanuts, who then serves the person with severe allergies, contaminating their food or beverages with the peanut oils, causing them to have a anaphylactic reaction, and potentially die as a result.

Exposure through a chain of contacts is hard to trace sometimes and for the most part, people don't think about these things. I know that after COVID, I do what I can to touch as little as possible in public places. I don't want your germs, or the cold, or the flu, or COVID, or anything else the random strangers that previously came through may have had and transferred to whatever I'm touching. No thanks.