If they implement a tier system (or simply anything that pushes a new echelon of higher end gear out every few patches), then most of these economic concerns will be nil.

The bleeding edge will always be the most expensive, and more than likely priced to 2-3 times it's worth (think of any tier IV materia used for relics). As new content and gear comes out, those old high sellers will depreciate in price, eventually making them available to the public at a reasonable rate.

This is assuming they follow a logical gear progression system. In FFXI (and we see hints of it in FFXIV), certain items were ALWAYS considered useful (Haubergeons, elemental staves, Scorpion Harnesses). In times of high inflation, these become out of the reach of new players. The only time prices go down is in the release of new content, or when a large amount of money leaves the system (SE deletes massive amounts of RMT gil). Some of the old time FFXI'ers may remember at the peak of inflation how prices were (5-10 million gil for NQ scorpion harnesses), and how low prices dropped (after the mass bannings and removal of gil, SH's were back down to 200-300k). A static system like FFXI's is very susceptible to inflation, whereas systems like WoW are much more stable. It's guaranteed that any power item, no matter what it's value (aside from collectible and fringe items, those may always be expensive), will eventually depreciate in time with new content and options.