Switching to real time requires overhaul on encounter basis. Realtime gameplay tends to have more encounters. It would require huge amount of work for modding team.
But why conflate two separate things? Balancing encounters is completely separate from creating a RTwP mod. I would be fine with a mod that just only enables RTwP combat and nothing else. All the talk of how encounters in TB games are "hand crafted" and "unique" and so on are complete nonsense to me. I have yet to see a single combat encounter in a TB game that was in way better designed than such encounters in any RTwP game.
They are not directly related. A common trait when people look for arguments to support a point of view is to consider that correlation is the same as causation - it is not.
Quite a few games have filler content in the form of an endless stream of unnecessary encounters that are largely the same, followed by some form of ( often ludicrous ) "boss" fight. This is just bad design, and has no causal relationship to how the encounters are actioned and resolved.
Although there is no causal link, there is a correlation with systems that have quicker resolution; because many games don't have anything much to do if you are not resolving encounters, it's easiest for designers to just add more. This then becomes an argument against the resolution system rather than against the designer.
It seems that a very large proportion of gamers like the formula of repetition+boss, and don't really like complexity, and don't really like to have to think too much. In truth, the games industry, like many others, is often risk-averse, so giving this to gamers is often much easier than designing in the complexity and variety we might like.
Whichever side of this "ragin debate" you might fall, the truth is that games like this one, based on complex rule-sets, remain rather niche products in global terms. So arguing too strongly with each other over details is a bit like flouncing around after a family tiff. I personally prefer something more akin to real-time, because I feel it's more appropriate for the medium and immersion. However, I do recognise that an effective game control scheme that retains that immersion is not usually possible, and that there are competing ideas as to what is the optimal compromise.
Regardless, I think the people making the game genuinely want it to be as good a game as they can make it, and although there are bound to be aspects that I will not like, I think BG3 will turn out to be a good game. If modding then allows each person's personal gripes to be removed, all the better.