This balances in very well with Preference centres.
I would obviously have a preference centre to better profile my recipients but also I would tie in an optout survey. As well as letting people opt-out you could also ask if they could tell you why and depending on their answers you might even be able to handle their objection. For instance, if they are opting out and they tick the “you send me too many emails” box, you could inform them of the preference centre’s ability to control the number of emails. If they tick the “relevant content” box you could tell them about the ability to tell you about what they want to be emailed about, etc. etc.
So as well as begin able to find out why people leave you could even try to give them the information that they were missing in order to help their decision. After all all we want is to give them the content they want. WE can’t make them want it but we can help them help us!
Obviously as long as you don’t make the actual act of unsubscribing difficult to the point of them hitting spam, you still need that one click unsubscribe functionality. Maybe kick it in after they have opted out through the confirmation landing page.
The key for an ESP would be to enable customers to personalise the form.
At the very end of the day, it is all about profiling and permission.
The better profiling you do the more permission you get!