Institutional churches may continue to exist in the future, but because they lost the trust of real Christians they will no longer be regarded as authoritiative. Real Christianity will be 'located' in the individual Christian, not the church.
What emerges to replace churches is likely to be something that can be expected to evolve in a bottom-up fashion; by the free choices of many individual people; to be based on human nature and to cater to widespread human needs.
The future ex-churches will - I think - be regarded somewhat as we currently regard venerable colleges, private libraries, or cultural organisations involved in history and the arts; that is, as being helpful for certain purposes. But churches of the future will not be regarded as organisations having ultimate authority; nor as worthy to demand or deserve obedience in the primary things of life.
Instead; the group-ish aspect of Christianity will be based on communities of trust and love - in other words self-selected and organically-developed mainly among family, neighbours and good friends.
A convenient word for such communities is the clan.
As Rodney Stark explained in his work on the sociology of new religions (e.g. The Rise of Christianity, 1996); this is how nearly all religions began.
Jesus's mother and brothers were among his closest group; other disciples were recruited from John the Baptist's followers; and John - his first recogniser and supporter - may have been a close relative ('cousin'). Lazarus seems to have been a beloved friend, and Lazarus's sisters Martha and Mary joined Jesus's inner circle (indeed I believe that Mary was Jesus's wife).
My interpretation of the Fourth Gospel is that is provides a detailed description of the 'clan church' that developed around Jesus.
Mormonism was likewise built from Joseph Smith Junior's family - with his Father becoming the Patriarch, his brothers having key roles. His wife Emma became the leader of the women's organisation.
Indeed religious conversion (at least when it is voluntary) has been found to occur mostly among
family, neighbours and close friends - among people who know and trust each other - while lasting conversion of strangers
and outsiders is unusual.
There are other examples - but the principle is that 'churches' began as extended family groups; and it may be God's will that this is what the church of the future evolves-into - the wheel coming full circle.
The church of the future will be no larger than such groups can be, in practice - which depends on many factors. Interpersonal relationship are, of course, primary in such love-communities. As well, such groups are smaller when there is a lot of geographical mobility; larger when people stay in one place and have a lot of social interaction.
One or two hundred would therefore be around the maximum size and most clan churches will be smaller.