Since many of ethereums transactions are from smart contract that use lots of gas, looking at the average transaction fee is misleading. Check ethgasstation.info for the transaction cost of a simple transaction. $0.08 will get you confirmed in the next block, and $0.02 will get you confirmed in 15 min.
That's fair, but that is just the cost right now though. The gas price necessary to get into the next few blocks has varied a lot over the past couple of months. Ethereum is definitely not immune to transaction cost difficulties as currently designed.
Most people don't want to use a currency for payments that is volatile.
Wouldn't that just contribute to Ethereum transaction fees getting higher, so they eventually have to remove it? They'd need to move to a cryptocurrency with better scalability (something eth might have in the future, but certainly doesn't have now)
Side-questino: Does ETH as a coin benefit from all the coins that are built on top of Ethereum network?
In terms of demand - yes, since all transactions must be paid for in ETH.