I've mentioned some of my paternal ancestors were Presbyterians, so I've a little knowledge of how that denomination has split and merged over the centuries.
In my youth there was movement towards the unification of many Christian denominations; it was being pushed by the National Council of Churches. It was generally liberal, based on Social Gospel, internationalism (World Council of Churches), etc.
Beginning in the 1970s or before it seems the tide has changed; instead of churches flowing together from tributaries into one big body of common belief, the water is rising and flowing back into the various tributaries.
The recent split of the Methodists is just the latest instance of secession movements.
There seems to be a broader phenomenon of institutions growing larger, then splitting. Back around 1970 we had a lot of "conglomerates"--companies buying up other companies into one big outfit. Even GE under Jack Welch was adding different lines of business. But there too the tide has turned, and selling off branches now seems the trend, at least in older parts of the economy. The new tech outfits like Amazon and Google have expanded. When and whether the acquisition process will reverse remains to be seen.