No Kaz I don't think 15 nm muddies the water at all. It more closely relates to current CTAF boundaries than 20 nm and reduces the problem of overlapping ones.
I operate in an area with several airfields with different frequencies including a busy mixed use airfield with up to 20 RPT movements a day. RPT as a matter of course make inbound calls further out than the recommended distance often to accomodate a RNAV approach.
Radar coverage is not available in the circuit area but ATC is contactable on VHF on the
ground.
It all works safely.
The biggest problem is too much chatter in my opinion.
I believe it is impossible to have all airstrips marked on charts but a low level multicom used sensibly would at least give some alerting of traffic leading to see and avoid without ATC frequency cluttering.
In my opinion the NPRM should not have linked together the two issues of multicom and CTAF in the one yes or no question.