I'll repeat what I said in another thread, but given the fact they made Rebirth in 4 years, and the sheer scale and amount of content in the game, I don't think any of this is surprising at all. Getting this game looking truly next gen and polishing it up as best as possible, realistically would've taken them at least a couple more years. They probably made the very deliberate decision to not spend 6-7 years on each new entry of this trilogy, and cut some corners. Otherwise you'd be looking at 15-20 years total for the development and release of the VII Remake Trilogy.
That said, I think the launch version of the game will be improved in *some* capacity. The demo is very likely an older builder because that's usually just how it works, and as is usually the case there will likely be a Day 1 patch also. There's probably a few different versions to the game between Launch and this Demo Build. That being said I'm not expecting a night and day difference, and neither should you.