You also have to consider the source of that survey. It's not a Christian website. You have to consider that they may be trying to sway Christian opinion in the US.
So take it with a grain (or a lot, lol!) of salt.
All surveys like Barna and Pew say the same thing. Christianity is in serious decline.
I think there are other issues contributing to the decline of members and in attendance.
Denominational churches are especially in decline. Most typically have an aging congregation with very few younger couples. Sometimes, no younger folks or families at all. I think there are a few root causes to the decline. One cause is television and the internet. Both have contributed to Christianity's demise. The two have exposed Christianity's divisiveness, judgmental and hypocritical side.
Just think of unbelievers, or even believers, watching Benny Hinn or Ken Copeland. Or watching high profile people like John MacArthur - the most hated pastor in America - or Paul Washer demonize nearly everyone and everything! People seldom hear an uplifting or encouraging word from these two. People see the badmouthing of others in the faith. Everything Christians do regardless of Denomination, or lack of, reflects on 'one another' especially to the unchurched and unbelievers.
Other issues with Churches losing congregants is the fact that people find most Denominational churches as boring and irrelevant. They conduct their services in the same structured way they have for 100 to 200 years! People, believers and unbelievers, aren't encouraged to return because everything is the same. They are not challenged -there is no depth - they aren't interested because it's always the same old - same old recycled stuff.
The topic that interests many unchurched is their interest in the future - AKA bible prophecy. Several churches I've gone to -
NEVER speak or teach about prophecy. I've not heard a sermon on the
last days in over 20 years!
Then the ones that do are often obsessed with pre-trib. Then there are those who believe 'prophecy' is useless because (I've heard it a dozen times) "there are more important things," OR - "How do we know what to teach since there are so many ways to interpret it".
Christians are destroying Christianity.