There are however many desktop as a service providers in existence that effectively host desktops in the cloud and allow users to connect to those desktops from a thin client or their PC.
I think as years go on the network access issue becomes less and less of a problem though. Plus, for home dekstop computers there is pretty much constant access.
Unfortunately sun tied the sunray to incredibly expensive/slow sparc servers. So while the terminals would never need upgrading, the servers where never competitive in the first place.
It's also silly to have expensive server ram filled with frame buffers and the zillion client processes that normal desktops have for weather, date, time, calendar, notifications, speaker volume, notification mirroring, etc.
A more enlightened approach would be something more like plan 9 where compute can transparently happen client side or server side.