You beat me to it
If BA wants to attract pax from the competition - and regular J and F pax as opposed to the occasional leisure J&F ones - then by default they are already likely to be high status members in another airline or alliance.
Most of these pax will be used to having a seat allocated at time of booking - something that BA will not allow them unless they are G or S status or on full-fares. So quite a few of these pax won't have this facility with BA until they build up S or G status.
So personally I believe the seating policy could be a major disincentive for regular premium pax to switch over to BA.
Does anyone know the ratio of J & F passengers on big corporate deals versus individual purchases? I thought most of those could pre-select seats? Don't shoot me for asking the question, but how many regular premium passengers are there that could switch who would not be quickly swept into the policy - heck if you're buying CW tickets, it doesn't take many flights to get to Silver anyway.