The article talked about liberal & conservative Christianity while the study dealt with religious liberalism/conservatism. C'mon journalists, use your terms consistently
Anyway, I think there's a few different groups within the "religious progressive" movement, each with their own trends; non-Christians, seculars, black Christians, & white Christians.
Non-Christians are definitely growing, largely through immigration, but they have a few converts as well.
Secular-Religious types are growing as I'd expect. Most people will never be able to embrace atheism. As many thinkers have said, Christianity will be replaced with some other religion, not science. This group contains the spiritual-but-not-religious people who dabble in different things. Their future is extremely bright.
I think a large number of
Black Christians used progressive in the economic sense. Most of the black Christians I know have more in common with fundamentalists than religious progressives. Ex: I recall a poll that showed blacks were more likely to be 6-day creationists. A large portion of this group belongs in the conservative group, at least in a theological sense.
White Christians are the last group. There are two trends going on here. As older mainlines die off and younger nominals leave the faith, liberal Protestantism wanes. However there is a smaller trend of younger mainlines embracing liberal Christianity full throttle (aka people like Scott & Nathan). While the nominal future for this group is grim, the younger generation will eventually be able to increase the number of bums in pews. I suspect mainline Protestantism will bottom out in it's share of the white population sometime in the next 50-100 years or so and start growing again.
Another point to consider is that white evangelicals are demographically about the same mainline Protestantism was in the early 1960's. Expect the evangelicals to continue liberalizing and begin to shrink over the next couple generations.