What you are saying is technically a solution, which has the advantage of creating a single page, but has the major drawback that it is not good marketing.
When someone sees a paid game with a free demo, what they think is that they must pay to play the full game and that for free it will only be an incomplete version and normally much smaller than the final game.
A demo is like a free supermarket sample, just enough for you to feel the taste, but it will not fill you up in any way.
If you want to "eat" you must pay. That is what a paid game with a demo says .
In this other model, a free full game is usually offered, and you have a more advanced version that is paid. Normally, as development progresses, the most recent version becomes paid and the one that was previously paid becomes free.
The demo model does not fit well here and the price per file model is often used more, but the same administrator commented in a post that it is best to separate it into 2 pages.