The question is, why should this "succession" matter? Different people understand different things under this term. First you need to to survey what people actually consider a successor:

1. D&D rule set?
2. Continuation of the story?
3. Gameplay mechanics?
4. Infinity engine?

For me personally, "succession" doesn't matter as long as the plot is interesting and gameplay is fun.

Also people succumb to their nostalgia where they remember only positive experience and forget about bad one. Try to play BG1 and BG2 now and you'll find out that not everything in those games is nice and peachy. I remember that I was annoyed that in BG2 every quest must have been resolved by combat. There were no peaceful solutions. You could pick a side but it was obligatory to destroy the opponents. Games evolve, things change. Devs should take the positive from the past and make new game better than before. Cloning experience won't work.

Worthy successor? Any great CRPG game (PoE 1 is not a good game in my experience, btw).