Now, my question is, why is this not a good deal? Isn't the potential reward infinite? Wouldn't it make great sense to spend a finite amount of money to get something that gives an infinite amount of money back?
1. Cex.io is just a company and it won't be in business forever.
2. As difficulty goes up, mining reward goes down.
3. Block reward halves every ~4 years, and there will be only tx fees for block finders in the end.