Watchtowers are not trusted parties and you do not have to be always online to be secure. There is a configurable period where you can broadcast a revocation transaction. Watchtowers can do it for you but it's designed so they cannot frame you. The penalty for broadcasting an out of date transaction is loss of all funds in the channel.

To summarize.

1. Attacks are extremely unlikely in the first place.

2. Watchtowers can prevent them if they do happen.

3. Watchtowers are trustless. They in no way resemble banks.

4. Without a watchtower you are not required to be always online to be secure.