I don't see why tx fees alone couldn't suffice. The total hash power would adjust, potentially lower, to match the total value provided by tx fees, whatever they may be.
Transaction fees are set by users, and not by miners. If you don't want to pay high fees, then don't submit transactions offering to pay high fees.
I'm starting to wonder if actually tx fees are controlled more by exchanges and web wallet companies, rather than simple users.
Correct but the problem is the way I see it a 51% mining majority is the death of a coin. When the hashrate lowers, it will become orders of magnitude more centralized than it is even now. ASIC's made all miners unprofitable except china, and in that case of fees only, only one or two pools would be able to be profitable thus further centralizing it.
Txn fees controlled by exchanges and web wallets? Exactly my point. We don't really have a say unless you spend a year downloading the blockchain on your own node. This would be diverted if the blockchain didn't support txn fees.