I see people paying USD 10 for an item, and then paying USD 30 in fee for the BTC transaction, and I realize that this is not sustainable. And its impossible for BTC to become a mainstream payment method if the price is increased with x3 when you buy something.
Where did you see that? I have never ever paid 30 USD in fees on transactions with 1 input 2 output or 2 input 2 output. If you are going to send $0.5 to 20 users, then you'll have to pay $30 in fees but in the situation that you describe, there is no way someone had to pay $30 in fees in this year.
By the way, transaction fees are actually problematic in bitcoin world. To my mind, Lighting Network looks promising and I hope service providers will adopt it and offer payments through LN.
Or another unpopular but my favorite solution will be to increase block size but the problem is that majority of people somehow don't like this idea and say things like it will increase the blockchain size, will be more costly to run node and so on but that can't be and actually is not a problem today.