Was pedestrian mode considered illegal before Crank vs Brooks? Would explain a few things...
I think it's more about changing assumptions. 50+ years ago, it was taken for granted that bikes, being vehicles, should behave like such. You don't push a car on the pavement when it's broken down, and similarly you don't push a bike on the pavement when you're not riding it. That kind of thing. And there was no great problem pushing bikes in the gutter, so why not?
Now, the predominant issues are that it's not safe to do that, and, as Cudzoziemiec points out, there are too many parked cars in the way anyway. So we've become used to bikes being pushed on the pavement, and no-one bats an eyelid at it (unless it's Regent Street on Christmas Eve). The law hasn't changed, but assumptions have.
The law didn't need to change, because it was never really clear on these things. There are quite a lot of areas where the law is unclear because no-one's needed to work out what it means. When the need does arise, and people feel it's important enough, there's a court case. If that is in a high enough court, it sets a precedent. But for bikes being pushed on the pavement, no-one's done that. So there's a general assumption that a bike ceases to be a vehicle when you stop using it as one, but not really much law to back that up.
From what I understand, Crank v Brooks was quite specific to the individual circumstances, so doesn't really have an effect here in establishing a general principle about when a bike is a vehicle. I did once come across someone who argued that you could always take off the front wheel (or whatever bits were necessary to make it no longer a practicable vehicle). However, since the need to do that has never been tested in court, neither, obviously, has whether that would be an acceptable solution in law.
