To be clear, they tried to keep the team alive. Originally it was 4 people. Due to the amount of pressure "from below" (as in, lots of requests to decide on how to do something in Nixpkgs, with whatever decision the team makes stirring up controversy) and the lack of communication "from above" (as in, the difficulty in figuring out the boundaries of their authority with the steering committee) two people burned out quite soon. The remaining two issued a public call for more volunteers and even reached out to people directly, but only one person was interested and AFAIU they did not end up joining the team.

Now, finally, both of the remaining members burned out, and with noone stepping up after repeated calls for new volunteers, the team has no other choice but to disband. No reason to keep a team that's literally empty.

As for their open letter, I think it was well-written and not combative or performative in any way. They pretty much just announce they are disbanding, explain the reasons why without putting any blame on any individuals, and thank everyone for their help.