I agree it's not going to happen, but as far as cost not being an option... the cost of the aluminum case on an iPhone 5 was about $48.
iPhone 6 is a little easier to machine so it is likely about the same, possibly a bit cheaper despite using more material (aluminum quite cheap; almost all of that cost is the machining of it).
Factoring in materials and machining costs, best guess you're probably looking at increasing the shell cost to around $60-65 for the case, and increasing the weight of the iPhone 6 to about 5.2 oz from 4.6 oz, and the 6 Plus from 6.1 oz to about 7.3 oz.
The increase in weight is negligible, and considering it costs Apple a little over $200 to make an iPhone 6 while they sell it for $649 and up, making that $220 isn't going to harm any bottom line. Especially after you factor in not having to pay for returned, bent units.
In return, both would be significantly stronger than the iPhone 5, nevermind the current 6.
Anyway, yeah, it's not going to happen. But I think moving to titanium would make solid business sense.
Not to mention that titanium would be a great selling feature!