This article is a bit confusing. Is this about being able to link Clouds together using IP VPN i.e. over the internet with WAN/LAN like characteristics? and then saying because this is possible it is reasonable to throw away WAN links between private data centres and replace with IP VPN? What type of WAN solutions are you talking about?
It isn't clear how it has been concluded that IP VPN over the internet is cheaper, just because some bit companies are doing it?? Are we talking metro, national, or international ? What kind of bandwidth? at least at national level i do not think this IP VPN is a sensible option to replace a WAN link. At least in the UK MPLS/VPLS type WAN solutions are actually a lot cheaper than "top tier" internet bandwidth. Install costs are about the same per end, extra nodes can be brought in the same way as IP VPN, and bandwidth is more than 50% cheaper - IP VPN needs a bunch of not so cheap stuff on top of this. Unless you are talking about a complete outsourced IP VPN solution then the modern MPLS/VPLS based WAN is a much simpler setup (low admin overhead) no VPN devices, no WAN optimiser/accelerator etc.
So if we are talking about cloud then by today's definitions a public cloud is only connected to the internet so IP VPN is the only way. Depending on what type of WAN solution you refer to this may also hold true, older point-to-point solutions might be more costly than modern Ethernet based solutions. Also depends on what countries you are talking about as cost is based on maturity and the current stage of the comms market.
WAN optimization kit have a place but a lot of the time used by network people as a bolt on fix to problems in software. And this is not always the smartest approach.