Demos are horrible from a developer perspective. There’s a decent Extra Credits video on the topic, Demo Daze - Why Don't Creators Make Game Demos Anymore? - Extra Credits - YouTube
And if every game had demos, there would be a giant chunk of potential consumers that would just play new demos every week and never buy any games. I personally would probably spend most of my gaming time playing various demos.
I really don’t think early access is a problem, at least for certain types of games. Typically games that would need a lot of play testing and have good replay-ability.
Darkest Dungeon
Nuclear Throne
Don’t Starve
Prison Architect
Kerbal Space Program
Factorio
They all benefited from long early access periods. Maybe just have more curation and content requirements for early access.