As mainline Protestant denominations and Roman Catholicism fail to attract as many would-be priests and ministers, MDivs will decline.
A better question would be "Why is there a priest/minister shortage?" for these denominations, and there are plenty of good answers to that question.
Yup. M.Divs are a requirement for (I think) most mainline denominations and the Catholic Church. It's definitely cheaper to become a pastor for a nondenominational one since there are obviously -- for better or worse -- no requirements. And it's the nondenominational churches which are, of course, "growing" (or declining slower, to be more precise) in influence for American Christianity.
One has to wonder if eventually the mainline churches will discard M.Divs as a requirement, or replace it with a church-administered training program, in the future in order to make that vocation more attractive to candidates. Same with the Catholic Church, but I'm not as familiar with how strict the RCC is when it comes to degree requirements. But there is already a shortage of priests, at least for most of the West, and I don't see that problem going away soon.
The European Equilvalent of the Mdiv for the Catholic Church is the Bachelor of Sacred Theology.
The Mdiv was only " added " in the 1960s as Catholic Seminaries became accredited by the protestant dominated seminary acreddiation groups like ATS.
So even if the Catholic Church drops the Mdiv, it will still offer the STB.
And besides, the Vatican mandates at least 6 years of seminary training for all seminaries around the world, so it's regulated from the top.
As mainline Protestant denominations and Roman Catholicism fail to attract as many would-be priests and ministers, MDivs will decline.
A better question would be "Why is there a priest/minister shortage?" for these denominations, and there are plenty of good answers to that question.
Counterpoint : The Overplethora of Priests post WW2 and before Vatican II has never actually been the norm in Catholic life at least. Before the 1940s-1950s, there was always a shortage of priests.