but what they are selling, just a piece of hardware with a software open-source light wallet installed on it?
what is the main difference between that solution, and same open-source software on some old laptop, that is not used for anything else, just to have access to your wallet?
why the Trezor is better (just asking, I do not think that it is not) from old laptop? and old laptop sitting somewhere is the basement could be not recognized as hardware wallet, or it would be harder than seeing Trezor as a wallet
what is a main Trezor advantage, in your opinion?
The difference is that your old laptop would be effectively an air-gapped wallet. This means it has to stay offline (not connected to any network) to be secure.
Together with encryption, that's a really solid setup. But the downside is that its not very convenient to use.
A Trezor (or other hardware wallets) have the advantage of them being used togeter with an online PC without the risk of losing coins.
Security-wise its not as good as an air-gapped wallet, but convenience-wise it is so much more pleasant to use.
You can connect your hardware wallet to an infected device without worrying to have your coins stolen since physical confirmation (in form of a button press) is needed to sign transactions.
That's what all good hardware wallets have in common. A downside of trezor is the physical security which can be avoided by using proper password encryption (which is also necessary with an air-gapped wallet btw).