55cm is true for R44 seats but it's not as huge a gap as it looks in that picture, the seat itself is normally about 53cm deep. So you only encounter less than 55cm as a problem in small cars where the front seat is nearly level with the edge of the back seat or further back than this.
For many forward facing seats it does say things like "Ensure the front seat is forward so that the child's legs are not in contact with it" which is usually sufficient for a 55cm gap. Also car manuals often say "Ensure front seat is forward when child sits in the rear". These warnings are usually buried in the text on the second page of the (car seat) manual before the contents, so people often miss them.
It's 50cm for R129 seats and you won't ever get smaller than this except if you push the front seat right back and it's literally overhanging the rear seat, which usually happens only in very sporty cars which aren't really designed to have passengers in the back.
The reasoning for this is actually seen in the regulations, it's part of the testing criteria - but it's not usually communicated directly to parents as it's simply assumed that 55 / 50cm is enough space in a standard car, and normally it is. Newer cars with the i-size compatible seating positions also take this into account with their design and the front seat physically will not encroach into the 50cm limit. And many higher quality car seats will be tested to tighter criteria.
I've never come across a rear facing seat which fits in a 55cm gap, have you? I'm pretty sure the smallest still take up about 70cm space.
The 55cm rule is true, and many popular RF seats are bigger than the most compact Swedish type ones, causing people to incorrectly assume that they all take up hordes of room, but I don't see why people perpetuate this myth that "if you can't fit a RF seat then you can't fit a FF seat" - there will be (very rare!) cases where it is literally true that a FF seat would fit but a RF one won't. And even if it's not physically a case of space, sometimes it's just practicality.
There is plenty of general car seat advice in forward facing manuals, especially if you buy one from a reputable, safety-conscious company like Britax, Maxi Cosi, Joie etc. True if you buy a basic car seat it usually has very basic/bare minimum instructions.
I actually think most people do RF until about 18 months now, which is much better than it was when my eldest was younger.
I do agree with disseminating helpful information about RF, dispelling myths, you can fit them in much tighter spaces than people assume, it's also possible (and very safe) to fit them in the front seat with airbag disabled a lot of the time, so there are ways around a tight space if somebody is inclined. And we tend to have a lot of assumptions about RF which are unfounded - many children happily RF to older ages if they are used to it.
What I don't agree with is making up new myths, scaremongering, or insisting on RF for everyone at all times as I think people should make the decision that is right for their family, preferences and budget. Certainly a 4/5 month old must remain RF, no question (but this is backed up in law anyway unless they are massive, plus 9kg FF seat models are dropping off the market like flies). I'd recommend everyone stick to the 15 month rule as an absolute minimum even if you do still have an older regulation seat which allows it from 9kg, because children under that age are especially vulnerable. And preferably longer than this really. Once you pass about 2 years though, certainly 3 or so, you're really talking about preventing incredibly rare events which in all likelihood probably aren't going to happen anyway. If you want to do that, if that is reassuring and RF doesn't cause a problem for you, then absolutely do it!! There are seats and solutions for almost every scenario and I will help you track one down if that is what you want to do, because it's usually possible, and if taking all the steps to prevent injury in an accident is what you want to do, RF is by far the most effective way to do that.
But if you don't like RF, if it's causing financial strain, if it's causing stress, if it's a pain in the arse multiple times a day, if it's making you/your DC miserable, it's really worth weighing that up against the very very small likelihood that you will even need the RF seat.