Of course the future can never be predicted 100%, but Markov is 35 now and how long will he realistically last as a top defenseman? How many 37 year old defensemen are there in the league now, or even in the past, who played full minutes and at full speed and were as good as they were at 27? Aging and Markov's slide is inevitable, but exactly when and how fast it will happen is still uncertain. Is it one week, one month, one year, two years, no one knows for sure, but we do know that it will definitely happen.
That being said, even if Markov does last two years he will then be 37 but the other GMs know that as well and what would they be willing to give up for him, especially those teams trying to build for the long term future. He will only be in short term plans anywhere he goes, or even if he stays, and that will be considered when looking at his value.