It's not necessarily the case that transaction fees are the only way, or even a viable way at all, to pay for security. Saying the block size must be kept small because that's the only way to keep transaction fees high is begging the question.

No matter how decentralized it remains, Bitcoin doesn't provide any security at all if it's not used, and if 99.99% of transactions must pass through other (probably centralized) networks due to block size limits how much security is Bitcoin actually providing in real terms?