As you said, miners can always accept a smaller fee, but (as you said as one alternative) I don't think the incentive to take a smaller fee now to potentially slightly increase the value of their hypothetical holdings sooner is there.
A soft fork which changes what the rules will be in 5 years time is more likely to be accepted. Once it is accepted, then it becomes a network rule.
Miners in 5 years who violate the rule risk getting their blocks rejected. This is especially true if the reference client rejects them.