The iPhone 4 still sells new, for $100 less. Same thing Apple did with the iPad 2. Not a huge price drop for something that became "obsolete". I doubt the 4S will be any different when the 5 comes out.
The same is true of screen size. The iPad 2 has pretty terrible resolution (honestly, text always looks slightly fuzzy, it's not just in comparison to "retina") yet people still buy that for fairly hefty money. Or at least hefty when compared to a non-Apple equivalent.
Unless that changes, I would expect a 4S to still bring decent money in two years. Plus a VM version will likely be rare, as it won't have been around long, and it is very expensive with no contract offsetting it.
At $550-650, you're paying for the iPhone experience first and then you're looking for the most economical way to utilize it. If the market has a ubiquity of pre-owned iPhone 4's that can be put on a GSM MVNO, it will make sense for fewer people to actually buy a $550 version from VM.
Maybe VM doesn't care how many of these phones people actually buy. Maybe they think that every iPhone sold is one less iPhone that Sprint has to buy from Apple. However, if they do care to make it enticing, the price will have to come down at some point in the next 6 months. The gap between VM and other iphone carriers in total cost of ownership will be far too wide and (even as a fan of Virgin Mobile) I don't think the coverage footprint makes up for that kind of premium.