To me it seems that just declaring them unspendable is the option that changes incentives/rules the least.  Giving the coins to miners changes the rules.

Imagine that this situation happened when 99% of the coins were mined and suddenly 10% of coins were going to be given to miners, giving them 10x as much as before this decision.

Doesn't make much sense to me.

Edit: The change that does the least change in incentives is to announce that there will be a window of spendability that reaches X years back at all times, where X > the oldest coin.  Then no coin is invalidated now, but a rule that keep uncertainty bounded has been introduced.