I remember the triangle things in the 90s as well, but they are terrible in practice, they make the belt less safe because they pull the lap part up across the soft tissue of the abdomen. This can cause children to submarine under the belt and/or cause internal injuries. It's really not worth it compared to a booster which will position the belt properly over their hips and collarbone.
Seatbelts are designed to spread the forces of a crash over the strongest parts of the skeleton. It's better to require car seat use than demand car manufacturers produce better seatbelts, because the majority of cars on the road aren't new, whereas car seats can be retrofitted to any car and are hence immediately useful. Seatbelt technology probably does increase all the time but making the belt height adjustable isn't very helpful when children are not only shorter, but narrower and wrigglier than adults.
Also, agree, it's only short 12 year olds who still need a seat. Typically this would be a very petite child who looks more like 9 and probably hasn't yet started puberty. They would need it because again it's not only height which can affect seatbelt fit.
witsender amusingly when the law came in in 2006, there were articles about children being embarrassed at having to go back onto a booster, and the head of whatever government department it is that makes the rules said that he suggested parents blame him. As though offended eight year olds all across the country had any idea who he was! :o