Its usually the case that exchanges that except fiat trade at several percent higher or more than those that just trade crypto. Take a look at CEX.io as another example. All coins there typically trade at least 10% or more than Bittrex, for example.
This type of thing certainly isnt a new idea. Ive seen and heard it discussed countless times, but Ive never actually heard of anyone doing it successfully or in a way that made it worth it. If someone has, Id love to hear about it.
I think it is difficult to due various logistics, trade limits, liquidity, wait time etc., and I think it is especially true now given the insane transaction fees and wait time, as mentioned above.
If you had a ton of capital to throw around and offset the fees, sure it would work, in theory anyway. I just doubt that at the end of the day it would very worth it unless you had hundreds of thousands of dollars, very liquid accounts, high trade limits, daily withdrawals/deposits etc.
Even then, you are competing with the few people who are or may be doing it successfully, and Ive also seen several bots that claim to do this automatically. There is a New Zealand exchange, Crypto- Something, Cryptnomator, perhaps, that even always you to do this right from their platform with several coins.
So yeah, in theory it works, in practice, not so easy.