Huh? Saradush had day & night cycles, you can even see it on YouTube videos. Night enables many unique things ... infravision, enhanced stealth, Ring of Shooting Stars, will-o-wisps, glowing potion bottles, peeping frogs, ladies/gentlemen of the evening, shadows, etc. Back to the original topic, Baldur's Gate is really two different cities, one under the sun and one under the stars. I do hope we get to see both!
IIRC, extended cycles and weather are flags you enable/disable in the area file processed by the Infinity Engine used in the Baldur's Gate games. I think people just get confused because there are a few areas with a preset time (when the scenario dictates such a behaviour).
The very same engine in the same game instance runs both Shadows of Amn and Throne of Bhaal, obviously it can deliver day/night cycles (and there are day night cycles in ToB).
This being said, imho, in BG3 we should first get a proper, accessible clock and calendar in the game before envisionning the implementation of day/night cycles.
The absence of a visible time reference is a flaw in any game claiming to be a RPG.
Back to the OP, yes playing in a 3D Baldur's Gate city is an old dream of mine, the primary drive to buy this game.