With more people focusing on webkit as a rendering engine won't it's quality and richness improve, and developer workload decrease?
Opera can still keep their own network stack with links to their proxy if they want - Google as it's own network stack on iOS
On the presto question I guess it depends on how profitable the business of porting it to other platforms e.g. less rich phones, games consoles etc. is.
But I've never cared that much about Opera so that undoubtably influences my opinion.