When you have a trilogy it's a complete story. Three films telling one epic tale inside a shared fictional world. (Pirates, Star Wars) Once you get away from that, the world becomes a franchise. This is why Apes started this conversation. I could be wrong but I don't think War is the finale of Caesar's tale. I believe there will be a 4th movie and he will be a key player in it.
There are plenty of Franchises that tell a complete tale in more than 3 films. Harry Potter is an obvious one, which is why I'm more comfortable now that there's Fantastic Beasts inside the Wizarding World franchise.
These aren't entrenched rules, only my entrenched guidelines.