This is anecdotal and subjective, and doesn't directly address the parent's claim (which was about “the long term”), but FWIW:
I've been driving a Tesla with “Autopilot” for five months now. Since then I've probably logged about 10,000 miles of use of “Autopilot” features – lots of two-hour trips between Eastern and Western MA; a MANC road trip and some shorter road trips; and lots of 30-40 minute commutes.
Prior to getting the feature, I was concerned about handoff. I was concerned that it would be difficult to force myself to pay attention during those times that I wasn't directly responsible for the steering and acceleration.
Instead, I've found that it's easy to pay attention, but that I shift to a more strategic mode: scanning further ahead and around for hazards. It's actual easier to maintain situational awareness than when my attention is split between what's going on in the next 10s–1m, and more tactical tasks such as manually tuning my lane position, speed, and follow distance. Driving with automation feels (I told you this was subjective) safer than manual driving.
Just as in manual driving you keep your eye open for certain kinds of situations (passing a pedestrian or car on the shoulder, erratic driver ahead) and are poised to go into action, I'm especially primed to take the wheel when these risk factors add up (as well as other areas specific to this particular vehicle automation: its uncertainty about which way to steer around a small traffic island at the end of a turn; its failure to see stopped – as opposed to slow – cars just after a sharp turn or lane change; its penchant – mostly fixed now – for taking highway exits). And being ready to shift more cognitive resources over to driving is such a natural part of being a driver in any case, that I imagine this is a common experience.
When autopilot is turned off or I'm in a car that doesn't have it, it feels riskier not to have that second set of “eyes” on the road.
There still could be risk compensation: I could be driving faster because I feel safer, for example. I don't really trust anyone's attempts to figure out a priori whether I or the population of current or potential drivers of automated vehicles are subject to this; instead, I hope we see statistics soon.
And – I have a few decades practice driving non-automated vehicles, that I can pretty effortless fall back to. I've swerved around emerging accidents a few times, and handled drivers coming into my lane and other near misses; I'm decades from having to consciously think about where to put my foot to brake. I do wonder about the hand-off to people who are just now getting their drivers licenses and are going to have almost all their experience in automated vehicles is going to work.
We may be entering a (hopefully brief) pair of uncanny valleys of vehicle automation. (1) Where younger drivers don't have enough practice to deal with crises. (This has always been true of new drivers, but they'll no longer be learning at one year of experience per year; and the handoff requirement is new). And (2) the parent poster's concern, that drivers in general will zone out when the car is driving. My anecdotal evidence, above, is that this isn't yet a problem with the state of the art, but that's just a matter of whether this is the year that happens.