How many religions with over one million members have collapsed in the last 100 years?
None have collapsed completely. Most Christian religions of that size or above have shrunk.
The best 2 examples of a significant Christian religion going to near collapse in relatively recent times are Worldwide Church of God (now under a new name) and the Christian Scientists (also known as the First Church of Christ, Scientist, and not to be confused with Scientology)
The Worldwide Church of God had about 145,000 members at one stage. It has less than a third of that now. They lost many adherents after they abandoned many of their more radical doctrines. That was a mistake the Borg would have taken note of.
The Christian Scientists for much of their history were larger than JWism. They had about 270,000 adherents in USA in 1936. In an eerie similarity to what may happen to the Borg, the Christian Scientists tried to become something of a multimedia organisation, producing the Christian Science Monitor, some television news programs, some journals, and a 24 hour news channel. It nearly went broke in the process.