This is where you wrong. Not all transactions will happen off-chain. LN isn't forcing anyone to use it, and it doesn't directly affect the base layer, so people will still be using it. If adoption will keep increasing, the blocks will always be full, and LN will help with smaller transactions that wouldn't generate much fees anyway, because no one would want to pay $5 fee for a $10 transaction, but paying $5 to move $10,000 is pretty reasonable.