I think that the credit to acoin exchange idea is a plausible possibility, (and I personally really like it). However, if the devs were to make this a reality they would almost definitely have an incredibly steep exhange cost of credits for minimal amounts of precious acoins. So, in theory it's a sound idea that could help with acoin scarcity, but in reality I personally don't think that it would help as much as the OP thinks it may. I made a post that presented a different potential idea if anyone wants to give it a look: https://forum.plarium.com/mech-arena/702_game-discussion/1760876_a-coin-currency-rarity/

I think the idea is good, but maybe instead of that (mostly to keep it balanced) more credits can be used to get a discount on specific things like upgrades, etc.  there would also need to be put a limit to how many times this can be gotten per day/week since the developers do need players to continue to spend money to keep the game running.