The problem with WM6.x is the interface. The likes of HTC tarted it up nicely but every so often you'd still end up at the WM interface, and then it gets ugly.
WM was fine in its day, and still fine for business use, but for a consumer it's not at all finger friendly, has an interface that'd work well on a desktop in about 1995, and doesn't score well on media handling. Last but not least, it doesn't have an app store, or even any easy way to find what's 'out there'. I installed about two apps on mine in two years, primarily because I didn't know what was worth installing.
WM will still live on for a while - particularly in embedded systems. For example, all our local taxis have boxes with what look like WM-based front ends, but legacy is by definition a shrinking market.