1. The Incubators seem to get their main payoff from Witchouts. So Familiars turning into Witches isn't particularly useful for them.
2. Fewer Witches means fewer Grief seeds so girls will Witchout faster. It also means fewer fights so the mortality rate from fights drops. Of course fewer fights means they'll burn through magic more slowly - ideally a magical girl will burn through magic faster than she gets it from Grief Seeds.
3. Magical girl teams are less likely to die in fights but have to hunt more often. They are more likely to Witchout through Grief starvation. However the results of one of them Witching out may be unpredictable (Pleiades Saints) or result in wasteful death (Tomoe Mami, earlier loop).
A couple of other considerations: Witches are also great for prompting contracts which might not otherwise occur. It's still possible to convince people to contract without them though, in for instance life-threatening situations. There also seem to be some indications that a magical girl who lives for a long time has a weaker Witch - I guess the question is whether the strength of the Witch affects the payoff obtained. I would argue based on Kriemhild Gretchen that it does, in which case it becomes a delicate balancing act of trying to make sure new magical girls can actually take down existing Witches.
Actually, from this viewpoint Walpurgisnacht is a failure, (at least in the first loop) because it simply kills lots of magical girls rather than causing them to Witch out. On the other hand, it does seem to draw Witches away from other areas, which is potentially useful if the Witch population is too dense elsewhere.