Six players would slow down combat. Not only would you be adding 2 more bodies on the players side you would need to add additional monsters to make combats challenging. Six players could mean as many as 6 more turns for combats and that's just too much.
You really need to explore the backgrounds in 5E. A dedicated thief is no longer necessary. With the Urchin background you gain proficiency in both stealth and Sleight of hand which allows any Dex character to sneak and pick locks/remove traps.
There are lots of options, and the smaller the party the more re-playable the game and the more strategy you have to employ.
There has been no combat in Early Access that I've thought that I needed two more characters to be more effective
Six players would slow down combat, decrease re-playablity, and lessen the strategy required to succeed. That's a hard no from me.
(If it's something you folks truly want you can Mod it in after full release.)
I've never had any of those issues that you listed in the original BG series nor would it be a problem for BG3. Players would still have options, still would be re-playable and still would have to strategize since you have more lives on the line. If players don't want to take more time during combat since combat has been sped up from DoS (a few extra seconds on an extra character or 2 would not take an eternity). If players want more of a challenge, they can still use 4 or less party members.
I understand your point with the Urchin background but that still won't replace a rogue. Also, that's now how I wanted to play my wizard and roleplay the character. Still have to have rogue in my party and I can't change the companions backgrounds.
It's an option not set in stone. I have to add, I thought Goblin party encounter was pretty goofy with 20 enemies and only 4 of my party members. You're right, it probably would've been easier for me have that extra party member or two but I would've killed them faster.