HBM is DRAM with TSVs in it (among other things) and we don't really know when it will become cheaper than GDDR5 or GDDR5X.
Whenever production of HBM/HBM2 devices scales up to the same level as production of GDDR5 devices in previous generations, that's when you get the cost advantage.
Main thing about HBM is that it can't be used with Kaveri and Godavari APUs.
It doesn't matter what can or can't be used with Kaveri. GDDR5 can't be used with it, either. Kaveri is a dead-end product. It's over, finished, finito, done. AMD isn't investing anything more in it. They pulled the 870k, 880k, and 7890k. FM2+ is EoLed except maybe for some Bristol Ridge Athlon parts.
But AMD has already made an investment in GDDR5. (Otherwise it wouldn't be on the Kaveri memory controller).
No, they specifically have not made any investment in GDDR5 for Kaveri. GDDR5 support for Kaveri was stillborn. They made no attempt at implementation. There's no evidence that existing Kaveri chips could even support it given a board that had soldered-on GDDR5. Just because the capability is baked into the memory controller doesn't mean it's tested or functioning. Nobody's produced production-ready UEFI samples to enable that functionality.
With this mentioned, one thing to consider is how long AMD intends to support GCN 1.x with driver updates? That will partially determine how long we could see GDDR5 boards released.
No, it won't. You will never see GDDR5 support on FM2+. The last thing you'll see on FM2+ is Bristol Ridge as an Athlon (which, incidentally, will not have any support for GDDR5 in the IMC, experimental or otherwise).