While the block reward stills gives a fair amount of bitcoins, it won't do in one or two decades. The miners will have to arrange their business based on the transaction fees. And now I'm asking; how will the system work if we all move to the LN? Sure, by opening and closing channels we broadcast transactions to the main layer, but if the majority behaved just like how I did, the mempool won't ever full.
If all on-chain transactions were LN channel opening/closing and a significant part of the population tried to switch to Bitcoin, the mempool would instantly be full and fees would skyrocket. If more LN adoption will mean more Bitcoin adoption, there won't be any problems. Another thing is, people would still be doing on-chain transactions, simply because they are more reliable (not that LN transactions are unsecure, but on-chain ones with enough confirmations are 100% solid). People won't be buying cars or houses through LN.