If they get Machado I'm fine with McCutchen-Quinn-Herrera (regardless of who plays RF or CF) with Williams off the bench. Certainly if you can trade Herrera for something big (or a really good prospect package) it's worth considering but he's not going to be a back-up, everyone will get their 400-500 ABs (except Quinn probably 'cause he won't be available).
It's easy to keep Quinn because his trade value is not that of a starter or top prospect, and his value to the team is still high as a part time player. I am still not sold on him as a starter even with the health issues. He gets 500 ABs and I'm not sure he would actually outperform Herrera (though if it's close he probably has a higher OBP and better defense, though less power). His final month last year was a .495 OPS (though of course, he was also hurt). Either way, whether it's a sophomore slump or health, I don't believe he's a full-time starter for a 2019 team trying to win it all.
Now, if they get Harper, sure, maybe trade Odubel, though it's easier to trade Williams at that point, you just wouldn't need him.