You don't think Larian has already done that?
Its well known that Larian increased the HP and lowered the AC of enemies in order to keep players from missing so often. How would they know to do that if they didn't already test the 5e rules? In fact, I remember reading a quote saying that they did test with full 5e rules but found that players missed too often which was the exact reason why they decided to decrease AC and then balance it by increasing HP. So they've already done it. And the "development" would just be changing some numbers around, the equivalent of having multiple difficulty settings which every game already does. In fact, the original BG had a difficulty setting called Core which emulated the core 2e rules, so its already part of the franchise.
Not to mention that 5e has already been developed by WotC since 2014 and has already had thousands if not millions of playtest hours by people actually playing it. Larian would just have to input the numbers. Let players have the option to miss more often if they want to.
And even if it did take significant development time (which I doubt), it would be a worthwhile investment because it would please a lot of fans. People like me who don't care about Divinity and who don't care about a sequel to Baldur's Gate. A lot of people have been wanting a video game implementation of 5e since its inception and that was one of the main selling points of thus game.
I never said that Larian never tested the game. However, when you're only testing something internally, you can lose perspective. In
this video talking about D:OS 2, Swen commented that the QA testers played the game so many times that it got too easy, so they started playing with a party of 3 instead of a party of 4 for added challenge, and so when players actually started playing with parties of four, the balance was off.
In addition, Larian has left many things the same as in 5e (HP & AC on player characters and saving throws, spells which are balanced around saving throws instead of attack rolls, etc.), and those things are based on 5e rules, not the Larian-changed rules, which means they are not properly balanced at the moment.
I'm not saying that Larian is incapable of adding a 5e mode, just the reality that it will take time and resources. And given that the game's current rules are obviously in need of balance, the simplest approach is to try going closer to 5e rules and tweaking from there, instead of going farther from 5e rules and needing to make additional changes to handle their previous changes.