I distinguish believing that good successor criteria are brittle from speciesism. I think antispeciesism does not oblige me to accept literally any successor.
I do feel icky coalitioning with outright speciesists (who reject the possibility of a good successor in principle), but I think my goals and all of generalized flourishing benefits a lot from those coalitions so I grin and bear it.
I roughly feel more comfortable passing the responsibility onto wiser successors. I still like the “positive vs negative longtermism” framework, I think positive longtermism (increasing the value of futures where we survive) risks value lock-in too much. Negative longtermism is a clear cut responsibility with no real downside unless you’re presented with a really tortured example about spending currently existing lives to buy future lives or something.