I saw a leaflet which has since vanished from the internet which insisted the main carriageway of the old bridge would only be open to public transport, small motorbikes/mopeds, and non-motorway farm traffic (by prior consent!) There's already a bus lane leading up to the roundabout at the south end and a "buses and taxis only" sign leading off the A90 which gives weight to this, and I think projections of the bridge's remaining lifetime rely on it. Presumably anything else will be fined, although without the threat of points it's easy to see it becoming more of a toll road for the wealthy.
All the plans I've seen suggest the direct slip road will totally go away and all access to the old bridge, for buses and excused cars, will be via the Ferrytoll roundabout, but I may be wrong.
From my observations, the exact location of the bottleneck has always been the "hump" of the FRB - everything seems to flow smoothly after that. I wonder where the new bottleneck will be.
Barring some disaster, so much has been invested in the FRB that I doubt it will be abandoned for some time - at least until the bill for keeping it up exceeds the bill for building a new walkway on either of the other two bridges.