I think that if we talk about mass adoption, it should mean that Bitcoin is largely recognized as a payment method. So I'm talking about the reality where you don't have to look for specific stores to use Bitcoin but can go to your regular ones and use Bitcoin to pay for groceries. I know, scalability. It's indeed a serious problem. Is the Lightning Network the only solution? Is it even a realistic solution?
Here's what I thought about to address the issue of transactions getting confirmed slowly. How about not waiting for them to get confirmed? When a person uses a debit card to pay for food, it's not like the transaction actually get's processed and the money is sent right away to the store. No, this process takes about a day or two, I've learned that about half a year ago when there was a mistake with a transaction. The money is blocked by your bank, and you don't see it anymore, but it's actually still there. So how about accepting that a person paid the moment after this person hit the 'send' button on the screen or something like that? What are the odds of a transaction not getting through eventually (if a person paid a normal transaction fee, of course) and being returned back to the sender's wallet? Is that happening that often that shops would not be able to take those risks?
Now, I realize it won't solve all of the problems, because if there are too many transactions that are being put through like that, the number of unconfirmed transactions will be building up. However, I think it could be a solution for a less significant adoption step or as a temporary measure. What do you think about all that?
If the merchant trusts you and gives you credit, you can already do that. Same as if he were to accept a wire transfer later from you. Alternative if the shop has online presence, you could buy online and then retrieve physically (or use delivery).
If the merchant trusts you he could also just accept your unconfirmed transaction. And i know there is a way to make a receipt of sorts no idea what its called, but have seen it appear in Electrum when paying Bitpay while there is still no confirmation.
Absolutely sure you can do things without LN. A bar can keep an old fashioned tab for example. Just think of any arrangement you would do if a wire transfer was needed (your cards don't work, you are without cash, whatever).
Perhaps someone will make some killer app for merchants, like the Chinese have with their weird electronic money (pre gov), and give them ease to accept Bitcoin.