A demo is not a guarantee for having commercially success.
But having no demo on itch, and only a paid game is almost a guarantee for having no commercial success.
At least this is my opinion on the matter.
You would need to be an established creator of games that people would buy from you without trying before. And even there, I have had bad experiences with buying sequels and such.
Maybe some very good game play videos or other forms of marketing.
I imagine two scenarios:
1. Someone finding your project by marketing. Word of mouth. Have previously played your games. Youtube. Seeing someone play it. Whatever. The point is, the user knows what to expect. And might be inclined to buy, before even visting the page.
2. Someone finds the project by chance. No knowledge. A paywall is a hurdle. A demo version lowers that hurdle. A web playable version or demo version lowers it even more.