Just a quick scenario: average Joe will use off-chain transactions and basically only companies will send on-chain transactions
In order for the average Joe to create off-chain transactions, he has to open two on-chain ones except if he's under the control of another party. But then, isn't the whole system ruined? 
That's my assumption.
I don't say that my scenario is more accurate than yours, as I said, in 100 year a lot can happen. But I would not be worried now about such scenarios.
But, the security will decrease a lot sooner than that. Sure, in 100 years each block will reward 32 sats, but what will happen once Bitcoin reaches $10-100 trillion market cap and suddenly the reward of the network cuts in half? Will the security drop by half or will the market cap double?It's sort of a dead end.