As for Thunderbolt only displays or Mac Pros I'm sure Apple will sell an adapter for £25 if they don't already.
There likely isn't going to be a "forward compatible" adapter.
Display Port monitors work in backward compatible Thunderbolt (TB) sockets. (e.g., the current DisplayPort Apple Cinema Display will work in the TB socket of a 2013 Mac Pro if it had one. Same for any other monitor that has a DisplayPort input. ). As a mini DisplayPort, the current mDP to DVI , VGA , and Dual-Link DVI adapters work too.
If looking for a monitor that will work independent of what TB move Apple makes on the next Mac Pro ( or not ) then a DisplayPort one works well. A DisplayPort 1.2 one probably even better over the long term.
Thunderbolt monitors need a Thunderbolt socket. They don't work in Display Port only output sockets from a computer.
Conceptually someone could take a TB controller and solely just use it as a transformer for the DisplayPort subset of the traffic. That is likely to run into the same certification failure buzzsaw as "data only" TB hacks run into with personal computer implementations. Chopping "half" of what Thunderbolt does off is a red flag. Some data only (really PCI-e only) dongles (chain enders) will fly, but a completely dataless one is extremely odd because it is so unnecessary given the range of other better matched solutions out there ( DisplayPort -> DisplayPort ; no dongle ) .
Apple is out of the display business. Apple sells docking stations that have integrated LCD panels in them. The key question folks should ask is why a Mac Pro needs a docking station. If need a monitor focused product go look for one from folks that sell them.
1 - .... but will any new Apple display be TB only?
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2 - Would the price go up or down on the current ACD if a new TB only display came out?
Thunderbolt makes for a better docking station connector than the multiheaded hydra ACD. So yes, probably will be TB only. Also very likely to get tweaked to match the new iMacs in design (laminated glass and thinned edges and perhaps VESA out of the box like the new option. ). Pretty good chance to get bumped to having USB 3.0 . A new Mac Pro is gong to need a discrete USB 3.0 controller too, so they can share that component.
If Apple takes the Mac Pro to Thunderbolt they'll probably dump the ACD. ACD price probably isn't a matter of up or down in that case. If talking about the short term, while the supply dries up you could see a bit of both over time. Down as most folks flush inventory. Up as product gets rare for the speculators who hold on to them much longer in inventory.
The primary target for the ACD though isn't the Mac Pro. It was the miniDP laptops. Given those aren't being sold new anymore the ACD market quite likely has been tracking down already. BestBuy already has them on sale. There may be a deeper clearance sale too later after the flush. (Displays bought with a mac system though get covered by Mac's applecare. Bought independent, it is an additional policy.)