Still, I agree it is obviously theft, but I still don't think we should take any steps to prevent it. If coins have been abandoned or lost or the owners are ignoring them, and they end up being stolen, then so be it. The last thing we want is for nodes/miners/devs/the community to unilaterally decide to make some coins unspendable or remove them from circulation.
You forgot that we aren't talking about some abandoned coins in a P2PK output. We are also talking about a much bigger amount of bitcoin (in total) in reused addresses, like a lot of the addresses in the bitcoin rich-list.
The decision also won't be unilateral, whatever the decision may be. It will be a fork that like any other fork requires support from the majority.