I would probably do a similar thing, demo on itch and full game on steam.
Can you elaborate your reasoning?
You can even give the Steam key with a purchase on Itch.
https://itch.io/games/steam-key
Some devs are concerned about local pricing or do not understand how pricing works and fear they are not allowed to publish on other platforms, without matching prices. But do not understand how taxes work and miscaluclate the prices. Or they just do not want to have the hassle of managing another site with payments.
But usually, developers are keen to release their game on as many platforms as possible.