5e first off is completely different from 2nd edition and 3.5/pathfinder which is like 3.75 both of which are based on 3rd addition rules. One of the big major difference that affects earlier additions and 5e is the reaction system as reactions occur differently in 5e than prior games and are a drastic impact on combat. Additionally with NWN you were controlling 1 character not 4+.
So your compairing a game where your controlling 1 character with a game where you are controlling 4 characters. You go back to 2nd edition reactions were not even a part of the combat system.
Additionally the way actions are taken in earlier editions of the game are completely different than how they are taken in 5e. As well as the mechanics of elevation on combat. First off your not taking a single action your taking action(s)/bonus actions based on class so not focusing on a complete functional turn based system first would obsolete that fact and there is no way to properly incoperate reactions into real time with pause. Because your reaction is not restricted to an attack of opportunity like it was in earlier additions classes using shields / spells can take reactions that differ from just an attack of opportunity. Such as Shield mastery allowing you to protect an adjectient ally following certain enemies turns ending in combat. Proper use of aoe vs real time with pause and being able to group characters vs having characters in different positions performing secondary actions is a limitation that is present in real time with pause vs turn based combat and is evident as alot of the pathfinder wotr players have demonstrated on multiple occasions having issues properly spacing characters actively in combat. Where there are players who can manage that to an extent the largest difference still comes in the form of reactions. 5e is very heavily reaction driven in proper combat sequences.
Additionally your seperating bonus action from primary action. Bonus actions are not limited to attacks with melee weapons they are also optioned with secondary spell casting which doesnt exist in pathfinder and 3.5 because every spell is part of the same sequence of actions so you could preset cast spell A in combat vs cast spell A then cast spell B should circumstance a b or d happen but not c because that system doesnt currnetly exist while I am sure there is potential for them to code it you would be looking at completely revamping the entire system and adding 1-2 years of code to the games release as your have to program a multi-senerio response. Older editions of DND did not have this.
Additionally again your reaction may need to be changed based on circumstances in combat again and this doesnt occur on your turn however you can respond to enemy actions taking use of this change in your reaction. This cant currently happen in real time with pause. There by limiting how you can respond and take actions in combat. Additionally drinking a potion does not take part of your action in pathfinder kingmaker / WOTR. Nor does it impact you significantly in other versions of the game where drinking a potion in 5e is part of a bonus action even if done from your inventory it still affects you in combat. Like wise with food.
There were feats as well in 3.5 / 5e that allowed weapon swapping to be done as a free action this doesnt exist in 5e and still has to be corrected as it is with the live release. As swapping weapons / Changing what your using in combat is considered an action in 5e. Much like im sure the correction to wizard spell casting still will be corrected with release as they finish more of the spells as again this is a core part of 5e which was the original design concept to the game to make it as accurate as possible. Not to say that a real time with pause options should not be available down the road to those who want it but larian has stated there focus is to get the turn based combat system correct and to make it as accurate to 5e rules as possible with minor exceptions for abilities like favored enemy and Favored terrain which do not translate overwell. Those mechanics do not particularly work well on the table top version either without the dm focusing very specifically in a way that can be beneficial to the class. Which is a limitation with a video game and its a poor element to begin with of the class. Real time with pause should completely and utterly be looked at as a secondary feature to add after the games full release when they have time to attempt to properly impliment it into the game. Much like turn based mechanics were adapted in king maker long after the inital release when they had time to properly code it your talking a feature that took longer than a year from the live release of the game.
And again the combat system I am not sure you could even properly code reactions even with that in mind. Not to mention the resources again it would take to control the actions /reactions / split actions of NPCs would be alot higher so you would be looking at double triple maybe even 5/6 times the resources being used during combat in order to maintain the combat fluidness with larger numbers of enemies in combat. The Graphics / Resource engine for bg 3 is much higher than it is in pathfinder or older systems and combat is alot more complexe in alot of ways. There are alot of differences in the combat system coming from someone whos played the systems since 1st edition the differences are very significant.
Short throwing alot of the rules out the window it just does not play the same way. At which point they already said that would not happen way way before the game was even announced. So you can forget that happening...real time with pause your pretty much best case if they decide to add it expect it to come in over a year after the full game release as the amount of code rework required would push the game back at least a year if not 2-3. So yeah... Not to mention it would probably come as an optional DLC due to the weight it would put on a system attempting to run those variables with higher system requirements. At which point 70% of the people asking for that wont be able to handle the system requirements to begin with.
------ Long and short of every thing above 5e combat functions completely different than older additions and there but the adaptation requirements would be completely different. You can argue pathfinder does it all you want the combat system is completely different.