If miners get a high subsidy from coinbase transactions, fee markets might not really develop at these early stages, even in PoW coins.
Also you did not answer to the "not every coin needs to pay miners anyways" argument... Fees on Ripple for example are more fighting spam than helping in any measurable way to deflate the money issued.

Amount of coins moved/transacted or "bitcoin days destroyed" kinda metrics might be more useful, but again not even applicable or even possible in all coins.

You talk about the money people HAVE to pay for fees, the amount people WANT to pay for fees is likely 0. If coinbase subsidies are high, few people HAVE to pay for fees, that's why they are ridiculously low in BTC for example. https://blockchain.info/charts/cost-per-transaction shows to pay for the current hash rate it costs over 60(!) USD per transaction! Only a tiny part of this is paid via fees, the largest part is paid via inflation.