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.
Why exactly do you feel it's so wrong to change the incentives in this fashion? If recycling was added now with a 100 year shelf-life on coins, there would be PLENTY of time to account for the added mining incentive coming way down the road. I know some are concerned that transactions fees won't be enough incentive to secure the network down the road and this would add to the incentive without increasing the intended supply of coins. Again, I no longer think this is needed but like to play devil's advocate...
This isn't really a big deal, but it doesn't give a predictive income for miners. If the early Satoshi coins are recycled during one year, it will be an enormous boost for miners and then a gradual drop. There is no reason to have such an irratic payback curve. People would invest in rigs like crazy that year and throw the rigs in the garbage the next year.
Fixing the transaction issue can be done by either increasing the transaction costs, or as a general tax, called demurrage. This depends on whether people think it is ok to have "freeloaders" own bitcoins without paying for the maintainance of the network, or not.