We don't owe Cam, Demario, or especially not Tyrann Mathieu a thing. They've been well paid by us, but they no longer perform at even a decent level. Mathieu in particular is a liability, not to mention his terrible effort week in and week out.
Alvin's skillset, even at an older age, is particularly valuable when it comes to developing a young quarterback.
There is no universe that this roster is competing in the playoffs. Not in spite of DD, Cam, and Mathieu, but largely BECAUSE of them.