MS have dx... they ll make sure its more used than opengl.
mac/linux people probably wish Opengl would get more popular, because it d help them sell the idea of their OSs and probably mean more games ect => more people useing their OSes.
If apple gets opengl to work well... I could see them stealing some market share from MS. Amoung casual people that are gamers ect. Linux is like... a chicken without a head, theres just no direction with it, theres 1000s of differnt versions, non of which are polished enough. Linux would need 1 clear direction everyone agreed on and all sticked with, and everyone worked on same version to get it polished (this 1000's differnt directions thingy is killing any chance linux ever had of being a competitive product).
I wouldnt bet against MS... dont think theyd give up without a fight. Directx probably isnt loseing grounds to opengl (ever) simply because MS will put alot of resources into keeping it at the top.
that said.. if enough of the market decide that opengl is the way to go... things might just go that way. It would take alot of the market standing together against directx for it to work though.