It's always interesting (if slightly depressing) to see where people's minds wander with things like this.
1) As has been stated, there are multiple (many recent) cases of people drowning in rivers and not being found for weeks, if not months.
2) There is no evidence to suggest abduction, murder or any other form of foul play.
3) Why are the people claiming there's no evidence NB fell into the river then jumping to the conclusion that she was abducted (or worse), when there's even less evidence for that?
4) What kind of evidence for drowning are people expecting to find? Some slippery/muddy boot marks along an embankment I could understand, but that's only one way to fall into a river - and especially when a distracting dog is involved - but other than that, what? She was well wrapped-up with the sort of warm, tight-fitting clothing you'd expect for a dog walker on a winter morning; not the sort of clothing that is likely to come lose and end up lodged on a tree branch or the side of a riverbank downstream. We already know where NB's phone was, so that doesn't need to be found.
As it is, instead of going along with the police's original hypothesis and simply waiting for what appears to be the sad realisation that her body will eventually be found (as so many ultimately are in these situations), we're looking for a 'shabby red van' and assuming there's some sort of local murderer doing the rounds. I think it's ridiculous, and I think some people are showing how (negatively) impacted by Netflix they now are.
It's possible to wait and be patient without jumping to all kinds of unsavoury conclusions. I saw Claudia Lawrence's name mentioned earlier, and I do remember that case from the time - I don't think this is that.
I haven't seen PA's TV interview yet but I did read the transcript (thanks for posting it a few pages back); the poor guy and their kids must be going through hell, but at this point I don't think anyone is really under any illusions.