Wholeheartedly disagree. Because doing it that way ends up in the massive mess of mechanics we have now. And this game wasn't marketed as DoS 3 set in Faerun. It was advertised, and milked for all the nostalgia, as a D&D game in the spirit of the Baldur's Gate series. Starting from an entirely different game system results in butterfly effects that ruin the adaptation attempt. Changing one small thing, like letting everyone Hide as a Bonus Action, carries down through dozens and dozens of different balance changes. Having so many surfaces chips away at HP in a rule system that isn't built to support that, whereas DoS 1 & 2 were developed, at their core, as being games where you went into every fight with full resources.
Starting from the other direction (the rules system you want to use) means you get to take advantage of all the balance built into that system. You get 6+ years of play testing, for free, and tons of additional content available. Start with the rules, then build the mechanics. I get that modding DoS 2 code is easier, and if they want to make the cheapest game possible that's the route I'd go. But if they want to make the third installment of the Baldur's Gate series and a game that will remain monumentally popular 20+ years from now, start from the rules first.
The fundamental DnD mechanics are implemented correctly (mostly) they just changed much in terms of enemy stats, advantage/disadvantage and action/bonus action need of skills/spells. I admit this has a huge impact on gameplay from a player perspective but I don't think it is hard to change or very time consuming from a programmer perspective (it's just the enemy database entries and a few lines of code). The real time consuming stuff is the testing after changing to original DnD ruleset when they see that certain encounter wouldn't work if you apply original DnD ruleset. But that's what EA is for. I fully agree with you that first step is to go back as close to DnD as possible and then look how they could integrate their own stuff like surfaces so it works with the rest of the system. But as I wrote I don't think it is so hard to do that that I would give up hope it is ever coming. They will have no choice if they want to ever come to an end with balancing issues...
