Using the many helpful comments given, and reviewing dealer pricing of the Apple II and II Plus in old issues of Byte magazine from 1979/80, I've come up with what I think is a reasonable explanation and timeline.
The most important factor affecting the price of both Apple II models during the transition period of late 1979 and 1980 was the rapid decline in the price of the 16Kbit DRAM's used in both machines. The time period coincides with the entry of the highly competitive Japanese DRAM manufacturers that would go on to dominate the DRAM market in the 1980's. The falling price of DRAM prompted Apple to both lower the price of their machines and gradually increase the amount of RAM included.
The original retail price of the Apple II was US$1,298 (with 4 KiB RAM) and US$2,638 (with the maximum 48 KiB RAM).
By the time the Apple II Plus was released, Apple would offer both the Plus and the original for $1,195 with 16KiB of RAM. But the price of DRAM's fell so rapidly that by 1980, an Apple II or II Plus with 48KiB of RAM was selling for this same price of $1,195.
And this leads to my conclusion that, indeed, Apple continued to sell the original and the Plus for well over a year at the same price with the same RAM configuration. Dealers throughout 1980 seemed to frequently offer both machines, usually at the same price, and usually with $100 premium for each additional 16KiB of RAM. So, $995 for 16KiB, about $1,095 for 32KiB, and the canonical $1,195 for a fully-populated 48KiB AII or II Plus.
I suspect there are 2 reasons that the systems sold as equivalents for this time period.
- For Apple's part, this is long before the industry had developed to a mature, just-in-time, model for supply chain and manufacturing management. Apple likely had plenty of work-in-progress Apple II's at the time the II Plus was ready, and their dealers likely also had significant inventory. Just clearing the channel would take some time, even if Apple immediately stopped ordering any material specific to the original AII. And they apparently did not halt Apple II sales immediately because of #2.
- It's also likely that Apple's dealers and customers were onboard with a slow transition to the II Plus. The dealers had their inventory, and the issue for users was that AppleSoft Basic had a slow start. It was plagued by bugs, and its incompatibility with Integer Basic created some difficulty to overcome with the entrenched user base. Customers that wanted to avoid the early growing pains of AppleSoft could save BOTH the expense of a Language Card AND the hassle of loading Integer Basic by just purchasing an original AII.
By the time the stock of original Apples ran out in 1981, AppleSoft was more proven, and ready to become the new "standard" Basic. The bugs that had briefly hurt its reputation in the early releases were ironed out in the ROM version that shipped in the II Plus. And Applesoft's replacement of the legacy Basic was cemented when they went on to more than double computer sales that same year.
Those who opted for an original AII during the transition probably ended up just adding a Language Card later. Or, they simply held out for the more substantial improvements that would come with the Apple //e.
