I am not sure that you fully understand what IMS offers?
Telcos need a standardised architecture to implement packetised voice. IMS offers this (and more besides - its bedfellows SOA/SDP complete the picture). Voice calls may well be "free" in a few years time but who will be supplying the infrastructure for these free calls? Customers are not interested in best effort services in the real world.
IMS has its flaws but I believe it has built too much momentum to be dismissed.
It's an interesting discussion, clearly I am pro-IMS but I am happy to accept that I could be completely wrong!