Converters tend to reduce the price of highly desired IOs and raise the price of IOs considered to be trash. Why?
Well, if you are looking to buy an LoTG +7.5, which many builds use, you might be willing to pay a few million to get it now at whatever level you want. But if the price goes too high, you'll bid on another IO -- maybe a different piece from LoTG, maybe from a different set -- and then play converter roulette.
Additionally, being able to buy with reward merits puts a ceiling on prices. Say you want an ATO, WinterO, PvP or Purple. Those can all be purchased at 100 reward merits from the vendor. That effectively puts a cap of 100 million inf on all the most valuable items. Why?
Because you can buy reward merits at an ATM for 1 million inf. Convert a 100 million inf to 100 merits and you have bought yourself the item of your choice. In practice, the upper limit is probably much less due to the existence of the converters. As discussed above, you might find a less desirable purple (or whatever) at a cheaper price and decide you'd rather roll the dice with converters.
IMO, the end result will be a better market overall. Nothing will reach the absurd prices they hit on live. Flippers will buy cheap trash and turn it to gold.