9. Not going forward with Green Arrow And The Canaries, Painkiller, and Wonder Girl spinoffs
This isn't so much a decision that the Arrowverse made, as the call ultimately came from above at The CW, but the mistake was so huge that it would be wrong of us to overlook it.
You might think that the decision to not go forward with planned spinoffs Green Arrow and the Canaries, Painkiller, or Wonder Girl didn't really have an impact on the Arrowverse, but the truth is that it did. These decisions were made by the original CW - not new owners Nexstar - and it indicated that the network wasn't interested in investing in the future of DC TV long before the sale of the network. In fact, if you look back, this might have been when things started going downhill for the Arrowverse.
When Arrow was ending, the shared universe was at a crossroads. Could it carry on without its parent show and an endgame in Crisis to work towards? The decision to launch Green Arrow and the Canaries created an exciting new opportunity for the creative team, allowing them to tell stories in the future using some of Arrow's most popular characters to do it. Then you had Black Lightning's popular antihero Painkiller at the center of his own potential spinoff, once again suggesting that there was more story to tell in a classic Arrowverse property. The inclusion of Wonder Girl was also exciting because, Arrowverse or not, it promised a young Wonder Woman-like hero on TV in her live-action debut - something that would have gone down extremely well with CW audiences.
The CW didn't completely move away from superhero shows because it greenlit the short-lived Naomi and Gotham Knights (which were both cancelled after one season) and had plans in place for a John Diggle-led Justice U (which Nexstar passed on) as well a potential Stargirl spin-off (which also didn't go ahead). But the decision to pass on two Arrowverse properties so soon after the culmination of Arrow suggested that the future of the franchise was in jeopardy - something that was ultimately true.