That's exactly what I see happening. Lift focuses on server-side owning "state" of a client's experience. The rest of the web is moving the other direction. Right now, if you compare Play vs. Lift I think Play embraces the notion that it is just a data-munger to a greater extent than Lift, hence my concern for Lift.
, Backbone, Ember, etc. These frameworks are driving the state of the art for client-side *AND* are more comfortable for web-designers. While Lift/GWT will always have a niche they fill well, I think the more mainstream web development will continue to push for client-side frameworks and servers that data munge. Heck, even PHP tutorials I've seen recently are all about generating JSON which really seems wrong (given how PHP was designed...).
So yeah, I agree with you, the web is moving towards client-side heavy, stateful, powerful apps. Anyone not riding the current will be struggling to stay mainstream, regardless of the merits of their underlying technology.