Libraries that are 20-30 years old and that people still use today are generally in continuous development over that time period, often with the same team (in a ship-of-Theseus sense). That means that they have been optimized for today's hardware for about 20 years worth of todays.

Now there are cases where libraries have ossified and there is room for a new library to come and eat their cake (look at ripgrep for an example), but there are also a lot of cases where they are going to be very hard to beat (a lot of boost is in this area, I believe, and there are many other examples in specialized domains).