100% yes. I charge my iPhone 11 Pro with whatever Lightning cable happens to be near me ... 5W, 8W, 18W. This includes my USB-C to Lightning cables, like the one it was packaged with, sure. But I have more USB-A Lightning cables just kicking around and they do the job whenever I'm passively charging. The only time I really care and seek USB-C is if I realize my battery is at 5% and I need to leave the house for an extended time. And if I find an outlet with a USB 3 fast charger block on it first, that is probably fine to boost reasonably quickly unless I REALLY need the full charge quickly.
100% yes. The lightning side of your phone doesn't care. Depending on your USB-A adapter it may be charging at 5W or faster. Older USB-A Macs, most PCs, some TVs, some power bars, iPad bricks and many third party power bricks all charge modern iPhones at a higher power than 5W with USB-A.
I am willing to bet most consumers have a mix of USB-A blocks around that they're using with existing lightning cables, not just the little 5W iPhone ones around, as per my above responses. And honestly, my iPhone 11 Pro came with a USB-C block but I still don't mind "slow charging" 80% of the time on a USB-A port if it is overnight or I am just working nearby at my laptop. Not much worse than Qi charing speeds on the slow end, and some USB-A can go up to 12W (although I agree most people probably don't have that wattage kicking around unless they're iPad owners.)
Many iPhone buyers will have a few lightning cables. One at home, perhaps one in their car or bag. These will all work just fine. Many of those buyers probably bought third party power adapters which are faster than the 5W Apple ones already, without even realizing it.
I suspect many buyers (not all) will probably just ignore the cable and use the wall adapter/charging solution they used before, assuming they're coming from Apple. If they're coming from Android ... decently high chance they have a USB-C adapter.
One time I was going through old boxes of Apple things in my household and realized half never had the cables even removed from the box. Yes, not everyone is as privileged to have so many Apple things, but I know my household will not need to buy any more adapter blocks on future phone purchases. I am 100% okay with this (and the lack of wired headphones.)