Someday - The remaining members will realize..that they were wrong and will have to figure out what to do with all these great and spacious buildings.
But..

Temples are NOT designed for Marriage - remember you go the Temple to be seal for ALLLLLLLL eternity. So..no grand hall gathering place for friends and family - no walkway to walk your daughter - -etc.

Cant be used for normal church services..no room for people to gather.

Maybe they will be used for baptism only? Seems a waste...one function only.

They can sell them to the Scientologists for retreats complete with hot tubs and baroque decor.

Based on my experiences, they can use them for Halloween celebrations, with a full bar in the celestial room.

They will be auctioned as McMansions by Sotheby's.

A major paradigm shift, from Houses of the Holy to Houses of the Overlords. And money--new money--makes it all happen.

Dibs on the Snowflake, although I don' know what the inside is like, but I don't need much, nice living room, large bedroom and bath. Hot tub, and a large home theater. I assume there is a kitchen but I really just need a microwave; )

High class erotic clubs.

Celestial room is the bar. Endowment rooms for videos with beds.
Washing and anointing...

Fun topic, it'll never happen. Still though, fun.

First, think of all those meetinghouses that will be going empty LONG before they shutter a temple. They will make FANTASTIC community centers, senior centers, urgent care centers all right smack dab in the middle of the residential areas. brilliant, brilliant, brilliant I tell you, fiendish I say!

Then, the temples. The temples would be different. The big old regional ones like Seattle might be turned into Class A+ Celestial Grade office space. The big ornate, unique ones would be fun condo conversions. The McTemples could easily transform into high end duplexes or a single family uberhaus. Remove the spire and the moroni and they don't look like anything religious.