...it really is more beneficial to them to try to cater to as many different groups of audience as possible, rather than aiming specifically for the BG fans. This game is their big step that truly takes them as a studio to an entirely new level. I don't see why they would want to turn around and try to please the BG fans now.
That aside, it's not just D&D rules and creature stats. There's the writing and overall atmosphere, for which Larian also has their own things.
I personally wouldn't call half heartily pressing DnD's class system and action economy into DoS design concept as taking a big step. On the contrary, that show me they are a one trick pony. Taking a big step would have been trying to make a game that doesn't immediately gets me thinking of DoS 1/2. Sure, the engine and art style they're using is a part of it but that's understandable and that is not all to blame. When I played a year ago, it hit me directly; oh, environmental tactics and surfaces, typical Larian, they even changed the cantrips to make them similar to the 1st lvl spells you get in DoS 2. And they really want those burning patches to hurt not adding saves to them.
Then, after feedback, they tuned that down and I felt; ok, now we're getting somewhere. There are still major issues like for example that they blatantly dislike the class system wanting to give class specific features to everyone because "in DoS the player can be whatever he wants", but they at least took a step in the right direction. And then, a month ago, they did it again, changed a spell to create surfaces though it shouldn't. Chromatic orb did have a similar effect in 4e but BG3 is (supposed to be) 5e. They have the knowledge and experience to make environmental tactics and surfaces, when created by the players, visually stunning. But it isn't needed to be forced upon us. Instead of having a barrels of oil standing in the middle of a room, seeing a small bottle of oil or an oil lamp standing next to an enemy is enough. The opponent can think; "ha, you missed!" then...*seeing the wizard starting to conjure flames in his hand*..."oh shit". We don't need a spell creating an electrified puddle of water included with its damage, we already have "Create Water" if we want a conducting element to make our lightning based spell hit several opponents.
There is no reason for it. Like you said, they do have other ways to give a lasting, personal imprint on the game. The writing (I love Larians tendency for comical relief, especially their snarky animals) and the art style. Not following the source material when it comes to stats and rules isn't "not aiming for the BG fans". It's disrespecting DnD fans. In a DnD game.
Also, it's never more beneficial trying to cater the larger mass. Instead of getting a sharp product that is really liked by one group, you get a bland product that is, accepted at most, by many. Which is also clearly expressed on the forums.
