Now, imagine an scenario where the Lightning Network becomes heavily adopted by people in the mainstream world.
The "mainstream world" you're referring to, will never adopt the Lightning Network, because they'll never adopt bitcoin generally. The mainstream people are enough with Visa, they don't need anything more. The Lightning Network helps
us. The bitcoin adopters. It is here to
revive bitcoin as a medium of exchange.
But it can't replace Visa, because besides the "mainstream" argument, Visa provides something LN doesn't. Instant transaction with nearly zero fee for one-time-purchase. If you want to just buy a coffee, it won't benefit you to use LN. You'll have to broadcast two transactions on the blockchain, that's a ton of money on fees. You'll use Visa, because after all, you don't want to make another transaction with the seller.
The transactions are instant but the community needs those channels to spend with LN, so, my answer is a big NO. Lightning Network can't replace visa.
For what community are you talking about? Once you make a LN transaction, any party can broadcast it at any time as long as the node is online. So even if one of them does not remain cooperative, the other can close the channel and every party will receive the final balance of their signed transactions.