Some paddlers do make agreements with local landowners over when and where paddling takes place.
The stability achieved the allows both groups to plan their activities and invest in their development. This is probably good for paddling clubs and commercial providers who need regular planned use of a defined area of water for club sessions and courses.
However those paddlers in less formal groups (or none at all) tend not to plan trips so far in advance, but rather need access to the right type of water at the time they wish to paddle. This could be locally, or further afield.
Clubs need regular (usually weekly) access to a local stretch of water for members to meet and pracytce plus places to visit for other types of paddling.
My own feeling is that having more agreement over use of water would enable better development of river sports - but that will only be achieved if those agreements provide suitable opportunities for the majority.
I do not believe it is necessary to have the ability to paddle anywhere at any time provided it is possible to paddle the right type of water at any time, somewhere that can easily be reached (e.g. locally if conditions permit).
It should be possible to group rivers in s locality and have up to half available for paddling at any one time and still keep others availble for fishing only at that time.