If you don't want to pass a car you just drop down the speed on cruise control and widen the gap between the car you don't want to pass and you. It really doesn't matter if you are 10m or 100m behind him if you are not going to pass it.
And if you get too far behind him you might bump up the speed a notch.
And if somebody gets in the gap it's even better because they are going to eventually overtake him just like they overtook you and overtaking one car at a time might be safer than two cars at once.
I can't say I've ever seen a disadvantage in leaving "too big" a gap between me and the car in front.
If someone wants to pass me and then sit up the other guy's arse, fine. I guarantee I'll be ten seconds behind them at the next road junction, or not much more. I don't think people realise that the relationship between road speed and journey time isn't linear - doing 90mph won't get you there appreciably faster than doing 60mph unless you're on a very long perfectly straight road with no junctions to turn off at.
The problem comes from having to mentally monitor an additional complex system (the ACC) in exchange for the meager attention savings gleaned from not having to closely adjust distance to the next car. That's "adjust", not "monitor". You'll still need to monitor distance to the next car either way, ACC is no excuse to drift off. For example, my non-Tesla ACC will randomly brake hard with no car ahead.
I personally use my ACC as an extra set of eyes. I set it and still manually adjust my speed using the +spd and -spd buttons on my steering wheel. If the car ahead changes speed and I somehow fail to notice it, my car will give me one more chance by automatically slowing down a bit. But every time that happens I treat it as an "oops" like airplane pilots do when GPWS (ground proximity warning system) alerts fire. It's great that automation averted the problem, but it became a problem due to the pilot's error.
Personally I tend to trust it and I only step in if I want to actually go faster and pass the slow vehicle (which usually happens, but sometimes I might wait 2-3 minutes "locked" to the other vehicle)
I always have to write a novel on HN. This is a very commmon cross country situation where I live: let's say I can't pass a car due to road conditions where the car's speed varies between 70 and 75 mph and my desired speed is 75mph. I know in 5 to 10 miles there will be a 1 or 2 mile passing lane I can use. In order to eventually pass that car I need to stick to within 150-250 feet of that vehicle so I can make use of the passing lane. If I just "drive slow" not only am I not going to be able to pass the car driving erratically, no one behind me is going to be able to either until the next passing lane in 10-20 miles.
And if you get too far behind him you might bump up the speed a notch.
And if somebody gets in the gap it's even better because they are going to eventually overtake him just like they overtook you and overtaking one car at a time might be safer than two cars at once.