If you're talking literal causality then Nasu hasn't thought of it, but thematically, the state of humanity in Notes exemplifies certain aspects of mankind that Nasu likes to put on a pedestal in his grand quest of modernism. Notes is kind of him subverting himself ahead of time, like 'what if everything I thought was good was bad'.
Regardless, we are shown a world in turmoil, and that kind of an environment brings out the very best in humanity compared to what you get in lostbelts. The collective enemy is clear, and all that's left for people to do is their very best. The strong presence of Alaya in Notes basically proves that Liners are still human. It's a twisted form of what humanity used to be, that lost a lot of its beauty and fun being alive, but it's twisted to the function of adapting and overcoming. It's the opposite of humanity becoming 'dull' like it tends to in Lostbelts, or in Tsuki no Sango. It's become way too sharp.
Notes isn't a doomed future. It's just one in an extremely bad state, but it's strongly implied that mankind can pull through sacrificing everything in the process. What Liners decide to do with themselves after they win, that might be a concern for the pruning mechanism, but the conflict that's going on certainly isn't.