If you were buying a Macintosh 128/512K, you most likely had an awful lot of money to spend on what was essentially a very pricey plaything. (Yeah, I'll probably get crap for that). Apple's marketing highlighted young white men, but I seriously doubt those kind of people could ever have afforded a Macintosh with accessories. ...So, if you think about it, the people buying Macintoshes were unlikely to be sitting around thinking "ohh, ahh, umm, well that extra $500 for another drive is sure a lot of money." Those who bought these machines were the early adopters of new technology - and they paid the price.
Apple targeted students and schools in particular and from my direct experience, I had a number of friends who realized just how amazing a step forward the Mac was and could barely afford the Mac itself, but scraped together enough cash to buy one. This put many of the accessories out of reach, especially if the external disk drive only meant adding a few more disk swaps. Even when the price of the 128K came down after the 512K release, making a Mac even more affordable to both students and the average user as well, the cost of the accessories remained the same. While you credit businesses as saving the Mac, it was only publishing-oriented businesses in 1985. What really drove the Mac were these students who saw the future and bought the Mac even when they could barely afford it with PCs being much less expensive, and continued to buy the Mac even after Windows 3.0 turned the tide – evangelizing their friends and family along the way. Schools and universities were some of the biggest purchasers of Macs and even they did not have external drives (I recall only one Mac in our 8 Mac lab had and external drive and there was always a wait to use it). Early-adopters, yes – they paid the price, but it was a price worth paying and sacrificing for. Businesses also had to make choices. There were many more mom & pop shops in the 80s counting their pennies and it wouldn't be the first time a corporation bean-counter didn't understand the plight of their workers. But yes, most of them would have justified the extra drive for productivity, though many just worked more hours. That doesn't mean the home user justified the expense and there were far more of them at the time.
You can still spend silly money on Apple machines, obviously you're getting quite a bit of kit for your money, but you could spend $7,000 on a MBP with loads of options, extras. And if I was spending $7,000 on a MBP, I would not be hesitant to pay another $500 or even $1000 if it really speeded up productivity, like a second FDD would have back in the early days.
First, $7,000 is a lot of money to pay for a computer today. I don't know anyone who would spend that kind of money frivolously – they would absolutely need it for high end work and just like businesses you allude to in the 80's who wouldn't hesitate to add an external drive, they would make the investment if it were necessary. But I also know people who can afford it who won't spend the extra money on an external drive for a MacBook Air thinking they can get by without it. These are the same people who won't spend money on a backup drive, or anything else that doesn't immediately appear necessary. That is a common mindset either now, or 25 years ago. Second, the original Macintosh is nothing like choosing which model Mac you can afford today. There was only one and it was the only computer of its kind. Period. If you wanted it and weren't rich, business or consumer, you had to scrape together your savings to get one. Forget inflation and forget cost of living, a dollar was much harder to make in 1984 than it is today, the middle-class was smaller, credit was not as freely available. If you didn't have the cash, it was hard to get a loan to buy it, since computers were not particularly necessary to most jobs in those days. The spending mindset going into the 80s stemmed from the previous 30 years of conservative spending, when every purchase had to be planned and sacrifices made for things you wanted. Unlike the last 30 years where teenagers today typically have cell phones, computers, credit cards, TVs in their rooms, designer clothes, jewelry, DVD collections, video games and spending money. I had NONE of that and not because we were particularly poor – we were middle class. It was a different milieu. That did not stop me from buying a used Mac 128K in 1986 after working my ass off all Summer to earn enough money to pay for it. Not because I was spoiled rich white kid who wanted the latest plaything, but because I understood how amazing the technology was and how productive it would be in my life. And yes it was a sacrifice, even at that price and yes buying a new external drive would not have been possible at that time, at any price. Piece-meal purchases were much more common in those days, you bought what you could afford and add to it as you can pay for it. Do you hear anyone talking about lay-away anymore? No, because instant credit and cheaper prices made that an unprofitable business model. Even if you did buy the Mac with an eye toward picking up an external drive later, unless you found you desperately needed it, there was much more of a sense in those days that one could endure the disk swaps rather than spend more money you didn't have, unlike today where a quick trip to Staples and a painless swipe of the credit card later, your life could be so much easier. That mind-set was simply foreign to most people 30 years ago.
So while you put forth a fine argument, much of which is valid to some extent within context, the overall picture requires a much deeper understanding of the world in which the Mac first appeared. What I have learned from this exchange is that historical perspective is indeed far more biased than I used to think it was.