Small note, miners do not decide whether a transaction is legitimate, they can only decide whether a transaction is illegitimate. They do this by not including it. This is a somewhat weak mechanism, limited by the degree of decentralization of mining.

Conversely, if they try to decide that a transaction is legitimate (that doesn’t follow historical rules) then everyone’s nodes will ignore it. This ultimately falls back to a social layer, but there is a strong bias toward not changing anything since everyone needs to physically update their nodes to work with the now-forked chain.