I'm saving bitcoin for retirement and will not move my coins for 20 years. You've just stolen my money.
I've timelocked some bitcoin for 21 years as an inheritance for a new family member. You've just stolen my money.
I'm a political dissident who has been detained without trial by a dictatorship for 15 years. You've just stolen my money.
I've been sent to jail for non-violent victimless crimes for 10 years. You've just stolen my money.
I've been working in another country for 10 years with some wallets safely stored at my parent's/other family member's house which I have not accessed. You've just stolen my money.
There are a hundred other reasons someone might not move their coins for 10 years. That does not give you the right to steal them.
And that's without even touching on the crazy idea of implementing a system which allows coins to be moved with providing a signature.
Initially, I also thought about all these cases (especially about political prisoners, since I'm from Russia, and my wife is from Belarus). But then I noticed that the 10-year timeframe is not the OP's fundamental stance but rather an illustrative example. So, let’s think about following questions:
1. What if miners start sabotaging block creation after 2140? Would introducing some coins that haven't moved since January 2009 help? (Similarly, in four years, coins untouched since February 2009 could be put into circulation, and so on.)
2. Some say quantum computers could crack private keys of UTXOs created in Satoshi's time. By the time such computers exist, nobody will be using the old algorithms (the community will be forced to transition to something else). But would it be fair that the UTXOs from 2009 go to the quantum computer developer? Wouldn't it be better to change the consensus regarding UTXOs made with old algorithms when transitioning to new ones?
In general, I can imagine situations where the questions raised in this thread will become relevant. So, this topic doesn't necessarily revolve around theft.