Just because a certain part of the audience won't use an option doesnt mean it shouldn't be included. The game has lots of options thst vast majority of players won't use. That's a weak argument. Adding the option of '5e Standard Rules' does not harm those who dont want to use it all. In fact, Im sure more people will use that option than those who would choose the hardest mode, play certain races, certain classes, or romance certain characters.
Tbf, by definition, adding 5e Standard Rules would take more dev time and actively harm those who don't want to use it :v) Because it takes away from other things that are more in line with Larian's vision and broadly appealing. It's also significantly more work than dialing numbers for a difficulty and adding a few barrels, particularly as most of that is likely done by script and based on a % increase. Races/classes are just part of the core game regardless so sort of a poor thing to argue in general, same with romance, and I'm not at all convinced a 5e mode would have more converts than any of the above. Regardless, it's clearly not what Larian wants or is going for, so it's sort of moot.
