Notably, you don't have to agree on how to design a system. Many such systems exist and you just have to pick which one you want to use. If you see the rules as fixed, you can enter the system that most closely matches your needs, and then trust that it's not going to change from under your feet.

Well, the properly decentralized ones let you assume that the rules aren't going to change. Many blockchains though will implement whatever changes their developers desire.