I may be missing something here, but why do manufacturers have to take on the liability?
Because the automobile insurers are unlikely to assume the full risk before they know what the risk is, unless they are forced. And as we know, nobody is forcing the insurance companies to do anything. The automakers are very much going to be putting their own pocketbooks on the line when they release self-driving cars, which is why you aren't seeing half-assed attempts at level 4 or 5 hit the streets now. If you were willing to accept human-like levels of collisions, fatalities etc., you could probably get that with current technology, but that's not a level we're willing to let the automakers be responsible for. Instead, we have to do it to ourselves. Rightly so, of course; they do have to do better if they want to be in charge. And as it turns out, they have to do much better. As a result, the automakers are not in a hurry to get fully self-driving vehicles on the road before they get ubiquitous V2V.
To really get the accident rates down, you not only need V2V on almost all of the vehicles which aren't self-driving, but you need for there to have been some time for the technology to shake itself out. You want V2V to get hacked and exploited before the self-driving car phase, when it will only confuse people and you can blame at least some of the resulting collisions on the drivers. I presume that we will actually get legislation demanding V2V retrofit into all roadgoing vehicles, and maybe trailers as well so that a lost trailer reports the fact, and where it is located, and what its wheels are doing. It will almost certainly include GPS and transmit the location at all times, and it will have to be connected into at minimum the speed sensor, throttle position, and brake switch. This is a relatively easy thing to do (owners of particularly vintage vehicles will have to install a throttle position sensor on the side of their carburetor, or the equivalent on their mechanically regulated diesel) but of course is a political minefield that nobody wants to step into before they have to. However, every automaker considers it a fact already, so you'd better get used to the idea. This may well have to happen before Level 5, no steering wheel and take a nap autonomous vehicles are allowed to travel at freeway speeds.
The guy's not an oracle, but I was recently watching an interview with Bob Lutz, and he was talking about the future of vehicle automation. He presumes that once we actually get up to level 5 vehicles, they'll actually mandate basically all the style out of them for aerodynamics reasons. There will be minor styling cues, and automakers will be free to play around with textures and minor shapes to accomplish different aerodynamics goals, but in order to make "road trains" efficient, the vehicles are all going to be shaped like minivans with flat faces. The lead driver takes a penalty from having to push through the atmosphere, but they get some of it back from the reduction of turbulence behind the vehicle when the following vehicle creeps up close behind them. He allowed that this might not happen right away or all at once, and that perhaps for the foreseeable future you'd be allowed to have a stylized vehicle as the social tradeoff for taking away your steering wheel.
Of course, this is all stupid. Stupid, stupid, stupid. We have the technology to make self-driving arrays of closely coupled cars right now. It's called rail. Something in between light rail and a roller coaster is what's called for. Ideally you'd implement it as a monorail (monorail? monorail!) which could run right up the middle of almost ordinary-looking cars using two or four electric motors for propulsion, split to the sides of the vehicles. Then you'd build vehicles which could drive on ordinary roadways at ordinary freeway speeds (they only need a top speed of 100 or so, forget all this ridiculous performance jazz) for a little while on a relatively small battery, but then get on a rail which would not only handle steering the vehicle, but which would also charge the battery. There could be stretches without charging, but ideally the whole of the rail would be electrified. You'd elevate the rail, so the footprint would be minimal. You'd use the network for long hauls, and then eventually you'd cut all the roadways back to just one lane in each direction and carry the majority of traffic on the rail, with the roadways being driven by enthusiasts, service vehicles, cyclists, etc.
To me, this is not actually the idea transportation network, which would involve less car-like vehicles. However, this is probably the only place we can realistically go from here which would actually be different. It preserves the car, and automobile ownership. A lot of people are very invested in those things, so a plan that does away with them is probably not realistic.