Some thoughts on stablecoins - it's been noted that they can make a great onboarding crypto for newbies. Until now, onboarding has been almost entirely dependent on Bitcoin. If new investors in crypto want to get some other coin, they have to go through BTC first, which may present volatility risk as you talked about. Once stablecoin/fiat currency pairs are firmly established on platforms, stablecoins can become the onramp to crypto.
However, stablcoins are only as stable as the underlying asset - stablecoins based on fiat suffer from longer term risk. The fundamental flaws of fiat are some of the underlying reasons for cryptocurrencies to exist. Can we anticipate a crisis years from now of fiat-based stabecoins losing value due to failing fiat?
Regarding Tether specifically - You talked about the cost of trading Tether in on-chain (off exchange) trades. I believe that there is cost premium built into its price pairs on exchanges, at least it appears that way on Poloniex.
Great insights as usual!
You're right about stablecoins being at the mercy of the inflation which affects the fiat currency it is pegged to, however while the price is not perfectly stable it's many orders of magnitude more stable than any of the cryptos :)