It honestly depends on the situation. I'm not talking about their performance as a moderator as much as how they stand on actually wanting to be a moderator. If it's something they've been thinking about for a long time, or if they just don't appear to enjoy being a moderator/a staff member in general anymore, then I wouldn't really try hard to get them to stay, since then it'd probably be better for them to actually quit. Of course I hate seeing moderators leave their positions, but I also love seeing new people joining the staff team, so I honestly feel like it works out for the best in those cases D:
But if it comes completely out of nowhere, I'm probably more inclined to suggest a DLoA to just get away from it for a short while, but eh, it really depends on who and why :p
But yeahh, no, I wouldn't try to get someone to leave in an attempt to take over the moderator position of the forum. Since when do people post blogs about them planning on quitting anyways :o