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.
Anything is possible, but I still think unlikely.
Also 5 years would be roughly after two more halvings, so the block reward (12.5 in ~2016, 6.25 in ~2020) would mean the reward would be down significantly by then, so would not doubt have less push back from people mining then as compared to a 25 BTC reward if the BTC/$/Euro value is similar to now. If the value has skyrocketed in fiat terms, then you might get more pushback against the fork even then, but if 0.01 BTC is worth $2-$3...who knows.
I am just not convinced that changing to a semi-continuous function will matter much in terms of network security since the halving time is widely known and miners have planned for it.
:-)