For the future, I do see BTC price increasing. Certainly the difficulty will sky-rocket when the next gen hardware pops out. It will also drop the cex.io cost per GH/s (or any other cloud hashing service).
Yes, it is that simple...
Rent the hardware at less than 50% ROI, you will not recover your 'rental' costs by selling the GHs that by that time have systemically fallen in price. Which it does every week, despite the harsh manipulation occuring daily in the cex.io 'market' to keep prices beyond 200% of their true value.
It's best compared to investing Dollars @ interest at a bank, and making 2% interest while experiencing a 4% inflation - that's how you lose money despite rising nominal numbers that may make you feel good.
Selling out completely before any significant price drop (natural or manipulated) is paramount, and we both know that is impossible to achieve every single time (which if you fail one single time, your investment goes the road of everything cex.io and mining power - it loses value).
If you were able to do that - you'd not be trading @ cex.io but at Wall Street and be a multi-millionaire by now.
A typical comparison between average own hardware and cex.io, is that your total investment with own hardware may or may not achieve ROI (depending on purchase price, BTC valuation development, diff and resale price).
Cex.io is different, as it guarantees you loss of investment (about 40-50% without trading and depending on time held onto rented GHs).
As a rule of thumb - the longer you rent&hold GHs @ cex.io, the more money you will lose of that rent investment.
If you were to re-invest everything you mine at cex.io indefinitely, you total net value would eventually approach near Zero - despite owning thousands of near-worthless GHs with them. This is how their game works.