"I had 6 at my table last night and they came alarmingly close to getting TPKed in the very first fight.
They only survived because I ruled that the pirates ran away when their leader was slain.
If you're gonna run a party of 4, you need a well-balanced party with the Holy Trinity otherwise I think they may struggle."
This is just hilarious. Sometimes I think that the more people you have, the more likely people are to be stupid. Also people tend to take unnecessary risks when they expect certain roles to be covered. If you are missing a healer and a tank for instance, you will be much more careful and creative to survive, at least this is my experience.
Obviously it will be different in a single player video game since one person controls everyone but it is still not necessary to have specific roles in the party.