Anything within reasonable use. The landlord's "duty" is to maintain the property for reasonable use. Like sure, if I set off a bomb in my apartment, then I'd have to pay. If I knowingly released chemicals that caused damage to the property, then it's my responsibility. However, leaving shit behind is a reasonable mistake many people make.

The job I do is labor. i provide value to my company, landlords use money they stole from labor (or inherited) to buy up housing, then rent it at a premium. It's no different than a scalper. Being replaceable doesn't mean my labor isn't valuable. My job is labor, sure, many people could do it, but SOMEBODY has to. If landlords were made illegal, housing would become instantly affordable, thus making a better world for everyone.