Without a sharp constraint on the maximum blocksize there is currently _no_ rational reason to believe that Bitcoin would be secure at all once the subsidy goes down.
Can you walk me through the reasoning that you used to conclude that bitcoin will remain more secure if it's limited to a fixed number of transactions per block?
Are you suggesting more miners will compete for the fees generated by 7 transactions per second than will compete for the fees generated by 4000 transactions per second?
If the way to maximize fee revenue is to limit the transaction rate, why does Visa, Mastercard, and every business that's trying to maximize their revenue process so many of them?
If limiting the allowed number of transactions doesn't maximize revenue for any other transaction processing network, why would it work for Bitcoin?
If artificially limiting the number of transactions reduces potential revenue, how does that not result in fewer miners, and therefore more centralization?
In what scenario does your proposed solution not result in the exact opposite of what you claim to be your desired outcome?
If space in a block is not a limited resource then miners won't be able to charge for it, mining revenue will drop as the subsidy drops and attacks will become more profitable relative to honest mining.