Later when there will be no block
Reward its expected that miners are paid by transaction fee.
But there is no way people are gonna pay 100$ Worth of btc per transaction.
So you are worried about things that
could happen in the far future based on assumptions from today. You can almost only be wrong.
Just a quick scenario: average Joe will use off-chain transactions and basically only companies will send on-chain transactions, worth millions of $ per transaction. Now think again, do such transactions worth 100$ to be confirmed?
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.