Having been a driver for half a century, I don't think that driving manners have become worse, rather I think that there is a lot more traffic on the roads these days, more motors, more lorries, and despite the road-building that's occurred, the roads are for the most part more crowded.
One thing that hasn't changed is the predilection of lorry drivers to overtake another lorry on a motorway and taking several miles to do it. When two lorries are effectively running in parallel at around 50mph, all other traffic is confined to the fast lane, which then slows down to the slowest motor in it. Newsthump once ran an article on a prize being given to a lorry driver who took from junction 3 to junction 9 on the M1 to overtake another lorry, and the satire seemed only a slight exaggeration.
Poor lane discipline is nothing new. I recall some 30 years back driving on a very empty M25 and a bloke was going at about 50mph in the middle lane. I was in the inside lane at 70mph, and to avoid undertaking him went over to the fast lane to pass him, and then returned to the inside lane. At that speed, on a clear road with no junctions, he should have been driving on the inside lane. When driving at the limit on a three-lane road or motorway with moderate traffic, I tend to keep in the middle lane, as there will usually be someone going slower along the way. If the traffic is light, I'll keep to the inside lane except to overtake slower vehicles, and I'll move to the middle lane when approaching incoming slip-road junctions just in case there is someone joining who hasn't reached a good speed.
One thing which I think has changed is that van drivers don't drive like the clappers these days, but just potter along. I always found it very tempting when driving a van to belt it somewhat, especially in Transits, which were really nippy. Perhaps van are different these days; I've not driven one in many years.
As for comments in previous posts about 'old drivers', I don't think my driving style has changed since when I passed my test at 18 in 1973.