Please falsify or agree with following:

It's not about the length of the game that needs time. It's the complexity. In demo, you use the same system as in the full game with more stuff (art, models, levels).

So if you make a demo, you already create the full game's system for A.I. actions (since in full version there is just more with altered behaviour), inventory (you use the same inventory in full game, just with more items), items (you have the basis for item feats, just more in the full game).

The reason demo is normally taken as "easy step #1" is because the lack of content blinds the designer from seeing the fact you are making the full game. Just shorter and limited <something>