Any relevant change to protocols will be implemented as a firmware update to Trezor devices.
Trezors are not 'subject to failure' as far as I know.
Paper is hardly more secure or long-lasting.
The issue he's talking about is that it gets harder to change a protocol when it starts to become embedded.
It's not that the protocol changes can't be released as an upgrade, it's more that there are many devices that will never get upgraded for various reasons. Think about stuff like home routers, for example, that never get an upgrade.
If the network stops completely, people would either learn how to do it, pay someone to do it, or buy another router
Bitcoin involves their money, so I'd think the incentive is there to get the upgrade particularly if they can no longer access their money. Upgrades that just add features will likely not get done.
Even if there is are many reasons to upgrade they will not get done if people leave the hardware wallet in a bank vault for x years. Many people want wealth they can treat like that.
No problem there. The bitcoin is on the blockchain and safe. The hardware device holds a copy of the keys, but they can be kept elsewhere as well for use with any kind of wallet in future.
I would have trouble believing that in 10 years time there would be no software around that could restore those keys given a current hd seed even if the protocol was changed. Maybe a minor speed bump to get someone to find the software and do it for you, I guess.