This assumes that you can actually maintain a 3 second follow distance. On some roads, you simply cannot, and an attempt to maintain such a distance leads to increased danger from all the cars that cut in.
Simply put: follow distance is not a unilateral decision.
That's what I mean with 'the road is too busy'. If you can't maintain a 3 second distance the speed needs to go down until you can. But you are right that this is sadly not something you can always do on your own.
I'm still convinced people decide way too soon that the 3 second distance is more dangerous. You might be right if people are constantly cutting in the moment you leave a gap, but if someone cuts in every 5 minutes you're still safer off with the gap.
Simply put: follow distance is not a unilateral decision.