^Eh good point
1. I don't see a big deal to move all your coins once in 5-6 years.
Of course not. From a perspective of a long term investor, I'm going to store my coins securely for a long time. Who's going to remind them by the way?
2. Agree. But if he has private key he can just move it. Of course this way everyone will be able to deduce some information about his activity but it's not like people don't expect him to return at some point. Just a little bit of certainty.
Isn't it his Bitcoin? Why can anyone choose to do anything with it?
3. It should be done only once - at certain block height block header will contain information about all aeged coins.
The problem is that, at that point, every single node, now or in the future will have to gather all of the information about the previous transactions and ensure that an address doesn't have any activity. That alone is hard, since Bitcoin does not currently store the block height(IIRC) of the UTXO in the database. This means that everyone has to rescan their blockchain. UTXO is added and removed even before the transaction gets into a block.
4. Let's say coins can be spent only if their age less than 700 000 blocks. This means that after block #700 000 all previous blocks become irrelevant. This means we will always have hard cap of blockchain size. We can limit it without fear it will grow out of control. We can say "Blockchain can never be more than 1 TB (this isn't that much tbh)" and make blocksize bigger without fear of centralization (scenario where normal users wouldn't be able to host full node because they ran out of available HDD space)
You can't make any blocks irrelevant. The whole goal of Bitcoin is for the full history to be available and verifiable. If you aren't allowing that to happen, you aren't running a full node at all since it isn't trustless anymore. Someone will have to ensure that the past information remains verifiable and accessible and that results in centralisation.
5. It won't take more computing power. Miners will only need to check if coins that are being spent in tx are younger than certain age. It won't affect POW at all and POW is what consumes computational resources.
Yeah, it won't take much hashpower by itself.
It's more about an issue with the morals than the technical difficulty. I fail to see how this would help in scaling Bitcoin since someone has to store the "irrelevant" history of Bitcoin as you've stated.