As far as traditional game, grouse and walked up guns go, you may well be correct. The performance is certainly there and a lot of people now seem to chose 20s on the little syndicate I beat on, and one bloke has had some success with a 28.
However, there is now a trend towards higher birds, where long barelled, reasonably weighty 12s coupled with a substantial amount of shot seem to be favoured. I would also suggest that a 12 of around 7.5lbs is the best tool as a clay gun, especially if you want to put 28gram loads through it. That could become a little uncomfortable through a 28, or even a 20 if you wish to shoot 100 bird courses every weekend or shoot a large day on pigeons.
It is also worth considering that a gun which you want to shoot wildfowl with needs to be able to handle a reasonable load of steel shot, another area where a 12 has an edge, this may become more of a factor in the future if lead is ever banned as well. Therefore, on balance, as an all rounder, the 12 is still best, but if you only shoot walked up days or a few pigeon for the pot, there is much to be said for a 28 or a 20.
A lot of the 20 bore o/u guns are actually much the same weight as the traditional s/s 12 anyway, so that may go some way towards explaining the popularity of the smaller bores.