Yes, the rules can be changed. But also there can be events that result in forks in the road. And some of the bitcoins will take a fork that leads to who knows what for those bitcoins. Most certainly those bitcoins will no longer be accepted back on the main fork. If enough forks occur people will want to know the provenance of their bitcoins in order to determine utility. That will require running confirmations back to the original mining for 100% certainty. How long will it take to perform these title searches? What sort of delays will it impose on transactions?