Fight 1 we were all so bad that we needed an extended rest. Fight 2 was a TPK. One character survived because of GM intervention. This was at level 1.
This is a little surprising. Was the DM pitting the group against level 3 or level 4 encounters? There are definitely risks involved then. But a level 1 or level 2 encounter should not result in TPK even for a party without a leader.
I've run some test games where we tried out a group of all strikers, and a group of all defenders. Both groups went through 3-4 encounters in the same day, of levels 1 and 2, without any big problems. The strikers were running low on surges at the end, and had a few close calls, but it was a breeze for the defenders. Obviously neither party had a leader, the closest thing was the paladin in the defender party.
A leader makes mistakes and bad luck more forgiving. And bigger groups are also more forgiving of mistakes and bad luck, than smaller groups. The DM does have to pay a little extra attention to encounter design for a small group without a leader. But it is far from a lost cause. As a matter of fact well coordinated efforts in such a group are very rewarding.