This reverts commit 69d572ca.
It's not an issue with sout-keep alone. The issue shows up if using
both sout-keep and #gather, and there is no easy way for a stream
object to know what its upstream stream (if any) is. Hence, let users
shoot themselves in the feet if they want, and otherwise assume that
people don't use #gather _in_front_ of #display.

good detection exist.

ILEoo, any idea why ?

Because there is no proper convention, there is no reason why the object couldn't be detached. We would need to vlc_object_find the input to be sure there is no problem. Let's assume for now that the input creates the access and is responsible for it.