No. This is what I hate about these iMacs. And especially more since this high-res display. You get a good, expensive display, which you could easily keep for 10-15 years, but are forced to throw it away when you want to upgrade the computer, after say 2-8 years. A Mac mini duck-taped on the back of a monitor takes about the same space anyways.
Are you sure about that? The current generation of iMacs can act as a standard monitor when connected to another machine. Up until a few weeks ago, I was using one that had a failed hard drive in it as a monitor for a desktop PC via a simple mini Displayport to Displayport cable connected to the PC's graphics card.
Probably not at this time - the display sharing feature has been dropped from the spec sheet, probably because the new iMac only has Thunderbolt 2 and thus doesn't quite have the bandwidth to drive a 5k display over that interface.
I think it will probably return when this model hits revision 2 and gains Thunderbolt 3 running the new display port spec that has adequate bandwidth to run a display of that resolution.
I believe this is also why we haven't seen an update to the standalone thunderbolt display yet - Apple will want it to retain the Thunderbolt connectivity and there's no specification available right now to run such a large panel over that interface (or any external interface I believe) without doing things like multiplexing two TB2 busses together.
Those wanting to use this thing as a monitor are going to be out of luck until TB3 arrives I fear.
There is a good possibility that the new iMacs can also be used as a monitor as well.