People who buy on Itch usually do so because they want the game DRM free, there are people who just don't like the Steam client.
It doesn't matter if you release the game on Itch and/or Steam, the people who will buy it expect a full game with full support.
Personally, people who only release a demo or alpha version on Itch and then release the final version only on steam. I don't buy anything from them, as I perceive that they only come to Itch to take advantage of getting views and not to form a community.
Note that I am talking about people who only release the full version on steam. Different from starting the game on Itch and then ALSO porting it to Steam.
I have no problem paying for an expansion, however, I do have a problem with the abuse of this model.
If you sell a game at full price, I expect a full game, if you then add additional content, I don't mind paying.
The problem I have is if you take a full game and mutilate it to sell as full price expansions and thus milk the users.
so.
1- NO
2- YES
3- Depends