Niche assembly: every survival must have its own unique edge of advantage. I guess I'm most convinced by this. The counter-argument is "how can every specie has its unique edge?", but it could also be that we have not yet discovered it.
Natural drift: if everyone is equally fit and the system is large enough, then they can all survive (for a long time). But how come is everyone equally fit? The response is "since everyone survived (as we observe), everyone must be equally fit".. Honestly I'm not convinced by this logic.
Social distancing: trees practice social distancing to defend enemies, so diversity comes in. If tree-killing pests are all staying on one tree and kill its offsprings around it (but cannot kill the original tree since it's too large), then this theory seems plausible. I have no idea if this is true in the forests though..