My whole workflow has been in 720p, including all the sources, so doing 1080p would mean starting completely from scratch.

Another good reason for staying with 720p, besides those already stated, is compression - to keep the same quality, the releases would have to become impractically large.

I should also mention, that the loss in detail in v1.0s was for the most part due to compression artifacts, which however weren't caused as much by the relatively low bitrate, as by me using Power Director for the editing, which had very limited encoding options and couldn't do lossless encoding. So even the DVD9 version of v2.0 will be far more detailed than v1.0, as can be seen in the last workprint, which was encoded with the same settings as the AVCHD version will be.