Building formations and angling at a merge (with each other and maybe eventually with PSL) would be the most principled and productive thing to do. But I would say that I think 90% of the people writing these letters are either burned out or have some personal liberalism that means they won't do it, not right away. To be clear I think most of the examples I've seen are the people who are burned out. But luckily bring burned out with comrades tends to lead to being not burned out so a long as people stay in touch and do some level of work, formations are possible.

Burnout is an underemphasized criticism of organizing, particularly in the US. A lot of organizational capacity is wasted on alienation and frustration that does not need to happen and could be avoided with structure and principled cadre. I've seen many cases where entire programs are built around squeezing labor out of new, young organizers to serve specific campaigns or strategies rather than trying to sustainably build something larger. Several labor unions in the US adopt this strategies with paid organizers, in fact. It's for the young who should not expect work-life balance or sufficient pay, so the logic goes, and if they threaten to themselves unionize, the union adopts the union-busting playbook! Obviously this is a petty bourgeois tendency, part of the important contradictions of non-revolutionary labor organizing, and doesn't purely apply to socialist organizations, but there are some common threads and mechanisms that must be understood and thwarted.