Plus without the ELRC or the third bore at Blackwell, Blackwall suffers enough that you want to try and divert traffic via Dartford (a tricky thing) if either end of the journey is beyond the M25.
I would disagree with that, I went from Streatham to the M11 regularly a few years back and using Dartford to get to the M11 would be madness. It was either Blackwall or back roads to Tower Bridge if a Sunday (and that was before the A12 was completed!).
I suspect shifting traffic from the A2 to Dartford is now a little more feasible as there are direct slips and you don't spend 15 minutes or so on a congested roundabout and will be easier still once the M25 roadworks are finished.
I suspect the amount of people who would be put off going that way by a bit of it being A102 instead of A12 are not statistically significant as the vast majority would either work the route out for themselves on a map or use satnav.
Knowing one or two people in the area I am pretty sure some will use it just to make a point even if it takes longer (the point being they don't want to pay a
tax on a tunnel/ bridge that was promised to be free once the construction costs were paid)