I agree with your stance on using smartphones (whether Android or otherwise) as cold storage wallets. It is not recommended since many people will not know how to properly prepare their devices for this task nor how to completely disable them from going online or connecting to other online devices. But I would just like to add to this part:
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Even hardware wallets might not be the absolute best for long term storage because they are also electronic devices that develop faults as time goes on. The hardware is just a tool, not the permanent home for your keys.
The best method to store your keys still remains a simple paper and pen backup. Don't try to play smart by creating new, unproven methods of storing keys. It just puts you in more danger. Stick to what is tested and trusted.
Yes, all hardware devices could fail at some point, including hardware wallets. That is why users are expected, and in some cases required, to create a proper paper backup when creating a wallet. I do not see any general disadvantage in using a good hardware wallet compared to a simple paper wallet. Because even a paper backup is not recommended to have only one copy, since it can be damaged or lost as well. Moreover, acting like paper is some invincible "God-tier" tech is a total joke. Paper burns, gets wet, and ink fades... even faster than an electronic circuit develops a "fault."
In the world of crypto, the seed is the soul, and the hardware is just the suit of armor. You need both. Use a hardware wallet for the high-IQ security features, but etch your recovery seed into stainless steel or store it in multiple high-security locations.