It's a common belief that doing cardio for weightloss loses its efficacy over time because that person becomes "efficient" at doing the cardio. As in, the cardio you do burns less over time so you have to do more cardio to make up the deficit. This a really common belief in the bodybuilding world such that a lot of bodybuilders won't do cardio in their off-season because they don't want to become efficient at doing cardio. Then they'll often start their season doing 90 minutes a week doing cardio, and by the end of it they're doing 7+ hours per week1.
So that begs the question, how "efficient" can someone become doing cardio? Let's say a person is capable of running a 5K in 30 minutes and will burn 300 calories doing so. If they run this 5K at the same pace every other day for six months, they'll become very good at it. It may even seem effortless. After the 6 months, they do their run. Will they burn 250? 200? Even less? It clearly would never reach 0. Any sort of movement is going to yield some kind of energy output. What would be the limit?
EDIT: In the hypothetical, the person remains the same weight.
1: I personally don't believe efficiency is the reason this is necessary, but that's not the focus of this question.