If you knew anything about Bitcoin Unlimited I am sure you would have come across its most fundamental aspect - moving the blocksize limited from the hardcoded centralised protocol layer to the transport or messaging layer thus allowing each node operator to choose the limit in a decentralised fashion so creating an emergent consensus.
In regards to your speculative economics, as I am sure you are aware, storage/bandwith/processing/validation and mining has costs. No one would therefore mine a transaction for free in 20 years because if they do so they would soon go bankrupt.
Bitcoin's design has an inbuilt long term solution. Reward miners early on - grow adoption - increase transactions to a "very large number" - charge pennies out of millions of transactions - profit.
Finally, LN may be secure for microtransactions. It is in no way secure for decent transactions. There are millions of ways it can be hacked even now that it only exists in speculative heads. Once it becomes hardcode implemented I am sure attack vectors will only increase much further.