"Insufficient" is unclear because there is no unambiguous definition of how much is sufficient.

In large part it depends on the decentralization of mining. If mining is decentralized then you only need a small (but still nonzero) incentive because any miner can't really do anything other than follow the longest chain rule. While raw hash rate attacks are possible (i.e. temporarily centralizing mining by incurring a cost), in a larger system they will have significant cost and will only succeed as long as the ongoing cost is paid.

If mining is highly concentrated by nature then you are really only relying on the weak linear security of the block reward itself, and maybe not even that, because miners (e.g., hypothesized Chinese cartels) have all sorts of perverse incentives.

Your statement would be correct if you added ", assuming mining becomes centralized as I have claimed it will."