I think wiretapping is one of the big reasons for the rearchitecture. Skype officially claimed they could not comply with wiretapping requests because of the P2P network as late as 2008 (http://news.cnet.com/8301-13578_3-9963028-38.html), and Microsoft was already working on wiretapping VoIP in 2009 (http://blog.tmcnet.com/blog/tom-keating/microsoft-patents-vo...).