Tesla of course claims that the HW3 cars will still get FSD at some point, but unless they somehow figure out how to bend light, that blind spot will continue to be an issue on older cars.
The fact that some early HW4 units will ship with the updated camera suite but not the Radar only further adds to my point that some users are going to be left with inferior sensing systems despite having paid the same $10k for FSD as everyone else. The whole "radar will be used to train and improve the vision" argument is just nonsense made up by Elon & Tesla fans. A properly functioning radar camera sensor fusion system will be superior in every way to a camera only solution. And there will be 0 Tesla's that actually achieve "full self driving" (ie. you going to sleep in the back-seat and waking up at your destination) until Tesla adds things like a cleaning system to their existing camera solution for example. The hardware is simply inadequate.
I'm saying that, as we see, HW4 and radar are two distinct hardware configurations: saying that there will be a large sore spot because HW4 cars have radar is objectively false as not all of them do.
The fact that HW4 and radar are separate configurations by name is not important. HW3 is also included in the mix (you can buy FSD on it) and it has totally different camera placement. So radar aside there’s still a difference in the sensor suite.
People who paid $10k and were promised “FSD” and future hardware upgrades have every right to be pissed off about this.
Installing the LiDAR unit in the front bumper would require replacing the entire existing front bumpers since there's no handy fake grille or whatever you can pop out and replace. You'll also have to paint the bumpers to match and blend onto surrounding panels since these cars have been in the sun long enough that a fresh-from-the-factory bumper will have an obvious color mismatch. If you don't, you'll have pissed off car owners to deal with and, in all likelihood, another class-action suit.
A roof-mount avoids that, but has its own issues. You'll needs techs to drill a freaking hole in the roof, mount the LiDAR unit, hope that they manage to seal the penetration well enough that water won't leak into the car, and then run cables through the interior. A whole hell of a lot of cars will leak, even if the work is performed to spec, so you'll have extend warranty coverage to deal with it. Plus, you've now got an ugly box on the roof that may or may not match the paint properly, whereas presumably the new cars will at least integrate the unit's lines into the body panels so it's not quite as obvious. The same thing goes sticking it on the hood. That's to say nothing of any hardware replacements and the fiddly bits that'll be necessary for refits.