So with the game being based on 5e, I feel like realtime will take all feeling of d&d out of it for me at least. This partially comes from having played 5e, and watched a lot of 5e content.
In combat, it is divided into rounds(6sec), in 1 round everyone takes a turn, order determined by your initiative roll.
During combat you get a MOVE action, up to your speed so depending on race or other variables 30+Ft
you also get to take an ACTION such as ATTACK, or something else such as DODGE or READY.
You also get a BONUS ACTION, this can come from Class features, spells or other abilities, they let you take an additional action.
along side that you also get a REACTION, certain special abilities, spells or situations can grant you a reaction, Attack of Opportunity for example would be a reaction.(this usually doesn't come into play until you're a higher level)
With real time, you lose a lot of nuance of the 5e rules, as a cleric you have to take more than 1 round to be able to move and cast a heal on a party member for example with real time, when the rules actually allow you to move and cast in your turn. In my experience with BG the enemy will still be attacking when I could have had the time to move and heal in a turn based environment.
Now I understand that people just want their classic BG experience, and that is fine, I'm sure they could do both with their engine, but If it is really based on 5e, it would be a shame to miss out on a lot of the combat rules. If you need to keep pausing to take advantage of the rules or to line up your magic, why not just have it turn based anyway?
I didn't play BG back in the day, I only just started playing before larians announcement. I often get frustrated wishing it was turn based like 2e it was based on. I'm only playing through with 4 characters, I can't imagine having to manage 6 characters pausing all the time when the game could have just been turn based in the first place. Now a way around this is with AI scripts, which is a weird thing to me. However it explains to me why people want RTwP, they don't actually have to control that many companions that way.