I think you have some wrong idea about stable coin. First of all, stable coins are for the liquidity of your investment and taking some instant profit. There's no other reason. If you are referring to the fee, it's nothing. You will not liquid $10 with tether. It will be a big amount where $4 is a tiny amount.
you are in a wrong idea too..fee will be a very big problem if BTC passes the $50,000 mark. No one will use BTC/USDT/Crypto if they need to pay $10-$20 fee per transaction.
I never used stable coins but as far as I know, exchange charges btc as fee when you send btc, eth when you send eth and USDT when you send USDT. If that's the case, how can the fee turn into $10-$20. Looks like I have missed a little to some part of your theory.