This seems to be a common narrative, that the savings have been passed onto the consumer. Is it really true? It's difficult to be sure, as there's no iPhone 12 that comes with the plug + earbuds bundled. However, there is one data point that most people don't bother to consider.
The iPhone SE (2020) base model launched at $399 in April. It still retails for $399 today.
In April, the iPhone SE shipped with headphones and a plug.[0] It doesn't anymore today.[1]
In the case of the iPhone SE, the price absolutely did not go down. All that happened was that Apple increased their profit margins.
[0] https://web.archive.org/web/20200427193132/https://www.apple...
[1] As part of our efforts to reach our environmental goals, iPhone SE does not include a power adapter or EarPods. Included in the box is a USB‑C to Lightning cable that supports fast charging and is compatible with USB‑C power adapters and computer ports. https://web.archive.org/web/20201023201007/https://www.apple...