SE5 will have a lot more improvement than just 3d graphics... For one thing, the diplomacy model will be greatly improved. No more rigid treaty types; we will be able to specify the exact nature of all agreements, down to what is traded, whether you can cross borders, etc. Read the chat session logs with Aaron Hall about SE5 to see a small glimpse at such new features:
http://se4irc.spaceempires.net/archives.php
Why would there be no demo? This is not some faceless corporation here... Aaron has no choice but to cater to the fans, or he won't make many sales...