I pulled this, because I feel like there's a lot of speculation that isn't in the text:
- In the Discworld Witches series, it is hinted in Lords and Ladies that the one thing Granny Weatherwax fears is the emergence of a witch who is more powerful than she is, who will compete for supremacy - and win. In the Tiffany Aching trilogy, such a novice witch does arise. Granny realises this, and neatly aces the dilemma by taking on Tiffany's teaching herself. This makes the prospect of a fight between them absolutely unthinkable, and Granny realises that while Tiffany will inevitably eclipse her, this is so far in the future as to resolve the problem: Granny will be dead or "retired" by then and the succession of Alpha Witch is assured without bloodshed or catastrophic destruction. In the meantime, Granny is free to express occasional seemly pride.
Firstly, Granny Weatherwax isn't really Tiffany's teacher. She teaches her some stuff, yes, as do Miss Tick, Nanny Ogg, Mrs Proust, and really most senior witches who have more than a couple of lines, but she's formally apprenticed to Miss Level, and subsequently Miss Treason. Granny doesn't really have the patience to formally train anyone, although she did her best with Esk.
Secondly, given that there is no record of there ever being a fight for the title of alpha witch of the kind described, it seems like a weird thing for Granny to worry about. One day someone might beat her at the Witch Trials, and she'd probably lose a lot of face as a result, but bloodshed? (Although admittedly, Granny would probably prefer being taken down in a rain of spells to being embarassed in front of her fellow witches.) The closest we get is the staring contest in L&L, and that makes very clear that by wanting to publically take down the alpha witch with a show of power, Diamanda has misunderstood witchcraft so badly that she's already lost.
And finally, there's no suggestion in the books that Granny sees taking on Tiffany, to the extent that she does, as being preventing Tiff becoming a rival. She's good enough at headology to realise that if someone ever does combine Diamanda's ambition with enough power, skill and understanding to make it stick, it's not going to be the hardworking sheep-girl who, if she is that powerful (and I'm not sure the books ever say she is), has never really thought about it because that's not how she thinks.
Edited by DaibhidCQuote discussion: I'm thinking the lines from Star Wars IV A New Hope between Obi Wan and Darth Vader. "When last we met I was the Apprentice, now I am the Master" my quote may be off though
Not sure about this:
In a meta sense, Ivy was already a much better-known villain than Woodrue when Neil Gaiman retconned him into her background. It's not like she was introduced as the student of the Floronic Man or anything. If that counts, then so does Batman being more famous than Wildcat.
And while he's not as well-known as Ivy, I'm pretty sure Woodrue is best remembered as a Swamp Thing villain.