The problem isn't so much cost in fees, it's just most people don't trust themselves with keys with the risk of losing all their wealth, they have more thrust in a company like coinbase. Others also don't even want to learn about all the technical stuff, they just want to hodl it and expect it to go up in value. About lightning, lightning is even worse than onchain, you have to run a node where your fund are not in cold storage, you have to manage channels and liquidity even in something like Phoenix try to tell someone that they can't reveice more than X because of inbound liquidity and to have more they have to pay to Phoenix to have more liquidity for one year and you still have to pay miners fees. I don't judge people that choose their tradeoffs, I think Fedimints might be a good compromise for these people if they know the custodians.