To answer the first bolded text: While you are not wrong about the nature of decreasing the block reward in order to put the focus on the transaction fees as the rewards for miners, the scaling issue of transaction fees does not really relate to that. What matters is where the reward lies, not the amount. The amount will always be related to the supply and demand of the available hardware in the network. If transaction fees decrease, miners get paid less and therefore mine less, which lowers the overall difficulty of mining and makes it more profitable for the remaining miners who are still in it. If they increase, the number of miners will increase with it as will the difficulty to balance everything out.
To answer the second bolded text: There will always be coins to mine. The Lightning Network still depends on miners. All it does is perform the reconciliation of may more separate transactions off-chain into a single final entry that once "closed" gets put into the blockchain. To get put into the blockchain, miners still have to choose the transaction and will still get paid a fee. The difference is that the fee will be much less than it would be with all the separate transactions, which is fine considering the supply (miners) numbers will adjust accordingly.
That scenario would still see to decrease the usability of direct bitcoin transactions. It means that everyone has to pay more fees entirely for the benefit of a small group of centralized companies paying less fees overall.
To give an example, suppose that we are in a future where miners only get paid in transaction fees and the lightning network is heavily relied upon to compensate for having never implemented dynamic scaling. Online merchants who want to process millions of transactions a day around the world utilize a system where one on block transaction opens the channel for the day, and another on block transaction closes the channel for the day. All transactions that go through their payment system is taken care of in one go keeping the fees low for everyone sending transactions between the two companies. However, the people who initially send the coin to those centralized wallets are going to pay extremely high amounts which would discourage independent wallet usage (and increase the risk of what happens when centralized companies go out of business). Meanwhile, the people who just want to send money to someone across the world wouldn't benefit from the lightning network at all since they are dealing with just a single transaction, so they have to pay more as well.
The reason why everyone is stuck paying more is because anyone with multiple transactions just processes one, which means that the cost has to be spread across fewer payers, and those with less transactions ultimately pay the most.
In a vacuum, you are correct. However, it is important to notice how a lot of the network traffic is, indeed, taken up by these small group of centralized companies. For instance, right now as the trading volume of Bitcoin increases, a lot of the transactions being sent and received through the network are related to some exchange disproportionately. Managing all of these transactions off-chain and then finalizing them on-chain would help free up space for the direct transactions you are talking about who would end up paying less fees because of less network congestion. The centralized companies
do benefit directly from this change, as you say, but it also indirectly helps users using independent wallets, which I assume would include most of us here.
As for what happens if the congestion in the network is actually caused by a big margin by direct transactions, I do concede that I am not sure how the Lightning Network could help in that regard. That being said, I certainly do not know all of the specifics of the Lightning Network, but you do bring up a good point.
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As a side note: If only most discussion threads could be like this one where people can actually discuss without spammers repeating the same thing others have already said, that would be great.