Well, the problem in the system is that the support team are humans. 
It’d be easy enough to make it clear in the admin UI what akrasia horizon each person is trying to hew to, but that still requires support to mentally re-calibrate what the akrasia horizon is in every single support interaction. If everyone’s is the same, 7 days, then there’s no chance of us making mistakes. If everyone has a different one, we have to check that, and even if the page itself calculates what dates we can change it from, we still have to remember to take it into account with everything we do to your goal, everything we say to you, etc.
When users are confused about things like the akrasia horizon, there’s also usually a standard answer: it’s 7 days. But if it becomes configurable, then we can’t just give the simple answer. We’d have to check the user’s account, or if the akrasia horizon is configurable by goal, each individual goal, to be sure we didn’t say something confusing.
Obviously, if I was convinced that a single 7 day akrasia horizon is a terrible and broken idea, I’d figure out a fix and help the rest of the support team get on board with that… but I’m not convinced of that at all. A 7-day akrasia horizon doesn’t work for everyone, but few things do, so standardised one-size-fits-all answers get applied all the time to all kinds of things… and work fairly well for most people.
I do think that most people haven’t actually thought it through and figured out what their personal akrasia horizon is, and I’m not sure I’d trust them if they said they did. So that also raises my eyebrows a bit about the idea of a configurable akrasia horizon; I think people are in general way too optimistic about their own willpower/immunity to akrasia.
To be clear, I’m not sure I’d trust me if I said I knew it, either; e.g. in the spirit of honesty I have to admit that my akrasia horizon on stuff to do with exercise is long. I’m… not quite lazy, but I have low utility for exercise-for-the-sake-of-exercise, so anything that feels “pointless” in the moment is not something I can stick to for long. Long-term, of course, there is a point to any exercise I can add to my routine – but I probably need to be made to make the decision to quit something 2-3 months before actually being allowed to quit. Admitting that fact is low-stakes now without a configurable akrasia horizon, of course… but would I admit it if I could configure that? I’m not sure I would. 7 days is annoying enough once I am done with a particular failed exercise plan (assuming it failed, as it usually does with me, because it began to feel pointless or always felt pointless). 
As an aside, if it was me who said in the past that support burden was why we don’t have custom weekends (as it most likely was), I’m not entirely sure why now, because it’s really no different to user-defined breaks. I guess again it’s an issue of flagging it in the system so that a) we’re aware of it and b) we actually remember to re-set the correct weekend. That I’m more sanguine we could figure out and work with than I am about a configurable akrasia horizon, though.