They can't do that as the whole things comes with the deal they make and also it will make them look hella petty.I hope Valve says something about the shit Epic pulls off in their GDC show.
The least they should do is close the store pages and forums down for those games. Seeing the developers using Steam as their marketing and social media platform only to go Epic exclusive a few weeks before release is disgusting.
Valve should have been more proactive in making sure they saw this coming but they didn't and now they are caught between a rock and a hard place. They can't afford to burn any bridges and it is clear the dev community is 100% behind Epic as I have not seen a single one outside of era dev posters coming to their defence.
Instead Valve took steps to ensure the user experience kept getting better and better on Steam but they did not anticipate this side swipe from Epic or any other potential rivals. Their insular nature and lack of focus probably contributed to this but personally I am glad that they looked into making the user experience on Steam better over cut throat tactics so I am happy to wait for any games to come to Steam. I value what they do more over some games and I trust them with my purchases more than Epic.
When EA jumped shipped and Ubi started their own store they should have started seeing the writing on the wall and made moves but hindsight is 20/20.