The Catholic loss isn't based upon people leaving for more orthodox denominations, their have loss their faith and have become nones for the most part. I don't think they are joining liberal Protestant denominations in great numbers.
But here is a Protestant take on Catholic fears:
The Collapse of Liberal Protestant Churches
Last week, 13 conservative cardinals delivered a letter to Pope Francis, complaining about some of the ways that his synod on the family is being run. They concluded the letter by saying, "The collapse of liberal Protestant churches in the modern era, accelerated by their abandonment of key elements of Christian belief and practice in the name of pastoral adaptation, warrants great caution in our own synodal discussions."
Conservative Catholics have a right to their opinions, but many of us in liberal Protestant churches would dispute their contention that we have abandoned key elements of Christian belief and practice. In fact, we have made changes in our churches because of our Christian beliefs, not in the name of "pastoral adaptation."
A generation ago, we began to ordain women because we read the Bible and saw the ways in which Jesus honored women. Female pastors have God-given gifts for ministry, and women are now leading some of our largest churches, just as they now lead corporations, hospitals, and military units. In 2014, the Rev. Shannon Johnson Kershner became the first woman solo senior pastor at Fourth Presbyterian Church in Chicago, followed by the election of the Rev. Amy Butler as senior pastor of Riverside Church in New York City, and the arrival of the Rev. Ginger Gaines-Cirelli as senior pastor of Foundry United Methodist Church in Washington, D.C.
More recently, many of our churches began to allow same-sex marriage because we studied the Bible and realized that the covenant of marriage is a promise-based relationship that has changed repeatedly over the centuries while always being grounded in love and faithfulness. Same-gender couples are joining our churches because they want the same Christian formation that has always been available to heterosexual couples. According to Matthew Vines, the author of God and the Gay Christian, a May 2015 Pew Research Center poll reveals that 48 percent of lesbian, gay and bisexual Americans identify as Christian -- a six percent increase from two years ago.
It is true that liberal Protestant churches have lost members over the past generation, mainly through low birth rates, failure to plant new churches in growing areas, and a nationwide decline in religious affiliation. But so has the Roman Catholic church. The fact is that many Roman Catholic laypeople disagree with the church hierarchy about birth control and gay rights, leaving the church badly fractured on key moral issues.
There is room in the Christian Church for conservatives and liberals, and I am glad to be part of a Presbyterian denomination that finds its vitality in democratic decision-making and alignment between clergy and laypeople on most important issues. While we members of liberal Protestant denominations have many challenges today, we can join Mark Twain in saying, "The reports of my death have been greatly exaggerated."
As for the 13 cardinals who wrote the Pope... they can criticize his synod on the family if they want, but people who live in glass cathedrals shouldn't throw stones.