If you run a node on lightning, you would know how much transactions you've forwarded, so you can make estimates, especially if you run a large node (say Bitfinex that runs their own nodes), but lightning isn't public the same way blockchain is.
The 1MB block limit makes it highly likely that there will be transactions in the mempool, you still need to open/close channels (even batched ones) and presumably large transfers for cold storage still happen on the blockchain, but then again it's very difficult to say what happens in year ~2140, we'll all be long gone by then.
At those time scales, it's pretty difficult to predict anything imo. That said, Bitcoin has been declared many times before and it'll continue to be predicted, it always comes back stronger.
It is also only 2140 if you believe that the market cap can double with the halvening schedule every four years. I see the upper bound (around gold market cap) in 3-5 halving cycles. After that the majority of the security has to be financed by tx fees
Miners will get an exponentially decreasing amount of block rewards until 2140, but even 20 years from now the (BTC-denominated) size of the block rewards will be substantially lower than they are today.
The price matters because they are BTC-denominated. If the price of Bitcoin doubles every 4 years, the security of the system can stay at parity with what it is today (assuming the real value of transaction fees stays roughly the same).
The 1MB block limit makes it highly likely that there will be transactions in the mempool, you still need to open/close channels (even batched ones) and presumably large transfers for cold storage still happen on the blockchain, but then again it's very difficult to say what happens in year ~2140, we'll all be long gone by then.