I feel like self checkout kiosks already solve this. Commodotized Amazon Go tech is a minute improvement in convenience compared to traditional cashier -> self checkout.
Minor compared to self checkout seems understating the impact depending on where you are and how busy the store is. I find myself waiting in line for a self checkout regularly. (Target and recently Home Depot.) Compared to just walking out, this would save at least 5 minutes for each visit in my experience.
Absolutely not. The difference in waiting times between cashiers and the self checkout lane is minimal (as it obviously should end up). Self checkouts require a reasonable amount of time to go through, even without a queue (when there often is one at the shops near me). Simply walking in and out would be a massive change from self checkout for buying a small number of items.
Self checkouts allow paying with cash as well. Even more so than regular checkouts. I tend to just empty all of my low value coins in to the self checkout knowing a machine won't be pissed off about counting them all.
The crucial difference is self checkouts displace the labor onto the end user with more or less the same delay as a checkout. This will remove both the delay and the checkout work task.