Nodes matter for different reasons, miners matter for different reasons. Bip300 is a soft fork, meaning backwards compatibility. It does not affect anyone not interested in sidechains. Miners often need to coordinate through softforks to respond to attacks or improvements that would take too long to get community consensus. They do not break bitcoin. They do not add more coins, they do not change difficulty adjustment, and other thinga that make bitcoin, bitcoin. Nodes matter for network affect, verification, ect.