While I can understand their reasons for shutting Upsource down, I think the very same reason will prevent users from migrating to their preferred tool-suite ("Space" [1]): users of Upsource very likely already _have_ existing tools for hosting repositories, communicating, issue tracking, etc.
It seems strange to me to expect people would abandon their existing toolchain in favour of Space, just to keep using Upsource.
[1] https://www.jetbrains.com/space/