When it comes time to spend your paper wallet, you need to load that private key into some kind of device (eg. Computer, phone, table) and sign a transaction. At this stage, you have to worry about the security of that system. Is it compromised? Does it have a hacked/buggy RNG? Will it leak your keys through a side channel? The trezor does the signing securely on the device. The keys never leave it and never need to be loaded onto another device.
ONE argument for Trezor vs paper wallet for the long term is what happens if BTC goes to 500k. Suppose you were the lucky guy that had an old paper wallet with 20BTC.....
To use them, you must load the entire sum into some computer and at that moment it is exposed to possible compromise. Because of the "NO CHANGE" property of paper wallets, the entire sum must be swept or imported into a wallet.
A Trezor with 20BTC in that situation, you could pull out say 0.01BTC and only that would be exposed to possible compromise.
I think it would be wise with the Trezor to verify the operation of the HD word sequence restore in one or another wallet, say the Electrum, then to save a copy of that particular program on a flash drive and put it with the Trezor in a safe. You proved it worked with that one wallet, not with any or some wallet that might exist in ten years.