BG1/2 and Planescape Torment even with limited technology were able to create a very detailed, believable and "mature" world. 3D games already existed at their time and they wisely chose what would fit best to portray their universe. In my view, they still (artistically) look good today.
BG1+2 are standart fantasy stories, but they are done very well. Calling them mature (whatever this means) is a bit of a stretch. The most famous companion is a guy who talks to his hamster. The game uses many known fantasy tropes. The main story of BG2 ( bad mage kidnaps my girlfriend) has been told countless times.
The games are done very well and I enjoy them a lot, but if you ask me for a very good mature RPG, I tell you to play the witcher series.
The graphics of BG1+2 are good and I still like to play them. 3D graphics existed at that time, but they were huge polygon blocks. Old 3D games look terrible today while old games in 2D or cartoon style still look relatively good today. I played NWN1 some time ago and in the beginning I had to force myself to play because the huge polygons are so ugly. On the other hand, I still like chrone trigger, for example.
Another example is Alundra for PS1. The first game looked like Zelda for SNES and it was great, I wish I could play it again. They used 3D for the second game and it looked terrible.
Regarding BG1+2 the devs did the right choice. The games are huge. Making a 3D game at this time was lots of efford and the result was still ugly. Having a good looking and easy to make 2D game (You just have to paint the background instead of building the world out of many polygons when number, size and color of them was limited) was better than doing an ugly 3D game.
Today, nobody make a huge 2D RPG unless they call it pixel art. A modern large scale RPG needs good full 3D graphics, so any new game will look different than BG1+2.
As for PST, regarding characters, story and setting it is one of the best games ever. Game mechanics wise it was terrible. They used a combat focussed system for a story focussed game.
From my current point of view, the gameplay system of Disco Elysium ( which I consider the best true successor of PST) is much better for such a game than DnD 2E.