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 could not have said it better. I agree 4-man party should be where it is at.