[1] Note this means the tail reward security of Monero will be very weak and insufficient.
"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."
I will argue my statement was correct as stated, because there are other parties with significant resources and incentives who may not be mining normally but once the hashrate declines to the tail reward cost, they can then decide it is easier to attack the coin.