If the blockchain can only process 10 tx/s,
That limit can be changed.
how will the tx fees be enough to secure the network when all the Bitcoins have been mined?
The amount of fees you have to pay can be changed. It is also up to the miners to accept a transaction with a certain fee or not. If a miner will only accept 1 BTC fees, but you can only pay .5 you are out of luck for that particular miner.
Especially if we consider the fact that most transactions will likely be off-chain.
Thanks.
What? Off-chain the way I understand it means: not on the blockchain. Thus its an internal action on some server, where you have an account and the person you are "sending" coins to has an account. Basically its just changing data in a spreadsheet. That has nothing to do with transactions fees as they are only needed (if at all) for on-chain transactions.
